Pinkie and The New Girl

The glass was back, but no one noticed it.

Marie-Sue was out for the day again, which meant the class was able to function normally. They'd mostly gotten used to her by now, but were still unable to run at one hundred percent in her presence. Probably never would. Pinkie didn't know why she was able to resist Marie-Sue's effects, but guessed it was related to whatever it was that allowed her to see the glass. Pinkie was convinced that the others would be able to see it, if only they'd look. They never did. There was always something to draw your attention away when the glass was present.

This time it had replaced the entire left wall of the classroom.

Pinkie wondered what the glass was doing here this time; nothing interesting was happening, they were just taking notes. Maybe she'd be able to get someone else to see it now.

"Psst, AJ, it's back."

"Gimme a sec, I'm a little behind the teacher."

"No, it might not... I can't ever tell how long it'll last."

"In a sec, I said."

Fine.

Pinkie stared into the opaque mist behind it, wondering what was there. Sometimes when she stared into it, she could feel it watching her back.

Then, suddenly, it was gone.

"Alright, let's see this... Damn it, Pinkie, I told you there aint no mysterious glass."

"No, I swear, it was just..."

There.

It was just there.

Had it disappeared because Applejack was about to look up, or had something been keeping her from looking up while it was there?
As Pinkie tried to rejoin the note taking she wondered if Marie-Sue would be able to look.

Be able to see.