Showers with doors or curtains on them. Privacy is a huge deal to high school kids when they’re showering. A lot of kids at my school used to refuse to take showers because they were too embarrassed. Because of this, there were a lot of stinky kids!
These already exist, but when I was in high school I had to lug around all of my books and it really hurt my back (our school was huge and spread out and there wasn’t enough time to stop at my locker) so I would have been thrilled to have a rolling suitcase/backpack thingy.
An anti-bullying device (not sure exactly what that would entail).
Delicious and healthy tasting food in the cafeteria (rather than either or).
Something like Fluther, where kids can anonymously ask questions, but there would be teachers, counselors and students answering the questions (and moderating the questions and responses so as to prevent bullying).