Pretty cool idea! Music is nice too, I enjoyed it. You've got me on two levels where I wasn't actually supposed to trigger the alarm.
The art is a bit inconsistent. Most sprites are pixel art, but the floor isn't. Some of the images have outlines, some don't. I liked the rats all over the school :P
Also, I think there's a missed potential here. Most of the levels involve just moving from point A to B as fast as possible. It doesn't feel that much stealthy.
Overall pretty solid game. I gotta ask - why are the bathrooms on the outside of the school? :P