It's time for a field trip (adventure bridge)

In almost no time at all, every foal had clambered up into the seats of the ‘bus’ which were clearly not designed with ponies in mind. As is the universal rule of bus seats, the foals all tried to sit with their friends if they could. Diamond and Silver, the spoiled brats they are, automatically claimed the first seat by the door so that if things went to Tartarus they could be the first ones off. The Crusaders took the one across the aisle to counter this ploy and stop them from bailing. The others just took whatever seat they could get.

While the foals were trying to figure out what the weird straps did, the strange substitute sat at the very front fiddling around with the buttons and levers. “Hmm, what to choose?” Ms. Frizzle mused. Her finger hovered over a panel of buttons with strange symbols on it. “Ah, here we are.” She said, before pulling a random lever, “Buckle up, class!” Sparks flew and to the casual observer they would see the bus shudder before stretching and spinning and shrinking down to the size of a kite. The end product looked like a toy version of a helicopter.

Inside, the foals were slightly dizzy from the spin cycle. “I don’t think I wanna be on this field trip anymore!” Apple Bloom said, her face tinged with green. Several other foals groaned in agreement, including Diamond and Silver, though they tried not to look like they were.

The only one who wasn’t sick was bouncing in her seat with joy. “That was awesome!” Scootaloo cheered, oblivious to her classmates’ wellbeings, “I wanna do that again!” During one bounce, she peeked outside and just stared for a second before shouting, “HEY! Everything is bigger!”

“Isn’t everything big to you?” Diamond said, unhappy that Scootaloo wasn’t sick like the rest of them.

Scootaloo was too much in awe to notice the tone and who said it. “No, I mean the school is like twenty times bigger!” Glancing around, she also noticed the new changes to the bus. “Is it just me or did this bus thing just change?” she said to no one in particular. The other foals started to look around too, amazed by the change in their surroundings.

“Oh ho ho, Right you are, dear.” Ms. Frizzle said, turning in her seat to face her, “This is my magic school bus. Great for every field trip a young class needs.”

“But what about the road?” Featherweight asked. “I don’t think we can ride there if we’re shrunk like this.”

Ms. Frizzle smirked. “As my cousin, Doctor Brown, always says, ‘Roads? Where we’re going, we don’t need roads’.” She turned back to the wheel and pulled another lever. Suddenly, the entire bus jerked as it lifted off the ground and soared into the air.

“We’re flying!” Pipsqueak cried out in amazement. The other foals gathered to their respective windows and stared out at passing ground below.

“I can thee my housth from here!” Twist shouted, pointing out in the distance.

“Hey, there’s our sisters!” Sweetie Belle said as she pulled Apple Bloom to the window.

“Oh, Celestia.” Button Mash said in growing horror, “My mom is getting broccoli!”

These comments continued as the tiny helicopter bus flew over everypony’s head, completely ignored by them. Soon enough, they reached their destination though the foals didn’t know that at the time. “Why are we going to Miss Cheerilee’s house?” Silver Spoon asked, “Shouldn’t we go to, like, the museum or something?”

“Because this is where our field trip begins, class.” Ms. Frizzle said as if it was completely normal.

“It is?” The foals asked in unison, completely confused.

The substitute nodded, “Mmhmm. Today, we’re going to look at how the body fights against diseases when you get sick, just like your teacher.”

“How are we gonna do that?” Diamond asked.

Ms. Frizzle turned to face her and gave an enigmatic smile, “Well, where else are we going to find the germs? Inside her body, of course!” She pressed another button and the bus shrunk down and turned into a fly. “Hold on, everyone.” She flew the bus through one of the open windows and navigated through the house to their target.

Meanwhile, the foals were enraptured by their new surroundings. “Whoa, so this is what Miss Cheerilee’s house is like inside.” Snips said, “There’s not as much chalkboards as I thought there would be.”

“Why does she have a picture of my brother?” Applebloom asked, making a note to mention it to her brother later.

Buzzing through the house, the foals found themselves staring at a very sick Cheerilee as she gave a loud sneeze into a now ragged tissue. “Wow, Miss Cheerilee looks really bad.” Sweetie Belle said.

“Yeah,” Snips added, “Lookit all that snot coming out of her nose. It’s like a slime factory.” Several other foals chorused their disgust at both the description and sight. It didn’t help that the last sneeze made a snot bridge between nose and tissue.

“Of course, Snips.” Ms. Frizzle said, “When you get sick, your nose knows it should try to help defend against diseases and works overtime producing mucus to catch and remove dust and bacteria that get in there.”

“Does it really work?” Snails asked, “I mean, if she’s sick, then why does it keep running so much?”

“Because mucus is only one of the few defenses your body has to protect itself.” She turned her head with a twinkle in her eye, “The real work happens inside the body.”

It wasn’t long before they flew into the kitchen area where they found Berry Punch cooking some soup. Pinchy hopped in her seat as she waved to the outside, “Hi, Ma!” They buzzed around and settled down on the soup ladle. “Hey, Ma is making her special hay noodle soup!” The delicious scent of the boiling vegetables wafted in through the windows, making all of the foals feel hungry.

“That smells like the soup Granny makes when my family is sick.” Applebloom said.

“Are the ingredients apples, apples and more apples?” Diamond Tiara asked with a sneer.

Applebloom rolled her eyes, “Pfft, No.” She conveniently forgot the time she tried to make soup for her family by boiling the apples whole to get a care-for-the-sick or cooking cutie mark. Somehow she still ended up covered in tree sap.

“Why do we need soup anyways?” Silver Spoon asked, “Doesn’t the medicine get rid of the disease?”

“Not quite, Silver.” Ms. Frizzle said as she fiddled with the buttons on the panel, “The medicine helps fight the disease, but the soup gives the body energy to fight back.”

Button Mash, who was curious about anything electronic and covered in buttons, peered at the teacher as her hands flew across the dashboard. If today taught him anything so far, it’s that those buttons plus a crazy teacher did not mean anything good. “Um, Ms. Frizzle? What are you doing?” he asked tentatively.

“Just getting ready for the next part of the lesson. Now buckle up, class. It’s time to go for a swim!” She flipped lever and the bus transformed again into a submarine, sliding straight down the spoon shaft and into the boiling depths of the soup.

“Does this mean we’re invited to lunch?” Snails asked no one in particular.

Snips gave him a flat look. “Snails, we are lunch.” It wasn’t long until the bus started to shake as they were tossed about in the delicious currents. At one point they found themselves falling again, but into a porcelain bowl. Outside the windows, a massive Berry Punch hummed as she ladled soup into the bowl.

“At my old school, we never fed teachers with students.” Pipsqueak said as the bowl passed into Cheerilee’s hooves.

Several of the foal’s eyes went wide as the bowl began to slowly tip towards the monstrous maw that was Cheerilee’s mouth. “I don’t wanna be food!” Diamond Tiara shrieked just before the sub dropped into the mare’s gullet.

Guided by the master hand of the Friz, the bus rode the soup and muscle contractions into the back of the mouth and down the esophagus, foals screaming out of fear and thrill all the way. Once in the stomach, Ms. Frizzle stood up and gestured around herself, “Here we are, class! Our first stop on this trip is the stomach. As you can see, this is where the food you eat gets broken down into itty bitty pieces before heading through the intestines where most of the nutrients get absorbed.”

“Huh, so this is what lunch looks like?” Applebloom said to herself, “I think I like it better before it’s eaten.” She watched as some of the carrots crumbled apart into mush.

“For once, I agree with the hayseed.” Diamond said.

“Wait, doesn’t the stomach have some super strong acid to break down the food?” Sweetie asked the teacher.

Ms. Frizzle nodded. “Right you are, Sweetie. This acid is designed to be strong enough to dissolve almost any food we eat. It can even dissolve some metals and other chemicals too.”

Silver frowned in thought. “So, if this acid can dissolve metal…” Her eyes widened in fright, “Are we gonna be digested?!” Some of the other students heard and spoke worriedly amongst themselves. Though it was cut short short by the tinkling laughter of the substitute teacher.

"Not to worry, class! We won't be staying here too long. We only have to wait a few more seconds." Ms. Frizzle gripped the wheel and pulled a few levers.

"Wait for what?" Rumble asked. Suddenly, the bus began to shake as the stomach fluids tossed them about, causing the foals to shout and scramble for safety.

"The walls are moving! We're gonna get crushed!" Snips shouted above the others.