Dude, I teach as well so I share your pain. Since I live in New England, there is snow, and therefore my school has a ski-racing team. I coached said team. Since we were leaving for races less than fifteen minutes after the end of school, the kids would bring their skis to school. The school's athletics department gave us a MASSIVE closet that we could keep skis, poles, bags, et cetera during the school day throughout the entire season. This room could easily fit 15 bikes just on the floor, let alone the walls. If I ever brought my new roadie to the school, I would totally press the AD about keeping my bike there rather than the rack outside. However, if I ever take my bike to school, I use my commuter and just U lock it to the rack. I do this for two reasons. First, the student body at my school is fairly mature with bikes and things (looking out the window today, I noticed at least 10-12 bikes in the rack; in a small community that serves two tiny towns and has only about 100 kids a grade, this is impressive stuff!) and students wouldn't mess with each others bikes let alone a teachers. In fact most students don't even lock their bikes to the rack! Some of these kids, coming from affluent families will bring a 2-3k DH bike (lots of mtn trails in the area) and not even consider locking it. Secondly, I would lock it up with the students because keeping my bike away from other students would cause lots of kids to ask and wonder why the teacher gets preferential treatment for HIS bike. I don't want that.

Therefore I think that you should check with your principal or AD or maintenance guys and see if there is a closet in an office you could keep it. Otherwise, weigh the options of locking your bike on the rack. Would it really be that bad? Oh yeah our school has a TON of cameras, so students are kept on a short leash with hijinks (they keep getting caught. The cameras are 8 for 8 so far this year!)