I would personally make the background a chalkboard to heavily emphasize the middle school theme while still having a dark background, which would allow you to have various writings and equations to break up empty space. That is just me though, I may also recommend light, greyish plaster walls and "giant" (relative to your character) windows/posters/hangings.
I have plans for a few levels in the school zone, at different places of the school. The giant classroom is one of them - and there will clearly be a giant chalkboard far in the background. Then, there will be the
"ink temple" where Bilou learns that he's the long announced "hero" who must save the planet. But first, he must bring back Booklet to the Library and have a talk with Pr. Harraps.
So says the scenario. At last, there is the
escape from Pendats jail and
epic fight against SqRt, the math book.
I want the library to be globally a dark and dusty place, hence placing the camera facing the wall (or the back of the furniture that holds the books ... but yeah, these could be just wall-mounted shelves, actually.
You should play (or watch on youtube) those levels, 'cause they are really cool.
I may have found a fair quality gameplay video on youtube. That will be for tonight. If you have the ability to do screenshots of the level yourself, I'm very interested by the glass flask just nexto the books in the CoI screenshot, as I lack a proper reference for pixelarting "inkjet" monsters. (unfortunately, it looks like there's only low-quality JPGs around on the Web)