The walls of our school are notoriously hard to get things to stick to. I have tried everything short of a glue gun (which we can't use because the walls are not a high gloss and the glue will take the paint off).
Finally I came across this great 3M painters tape for hard to stick surfaces (it is easily identifiable by its green color). I have not had a problem since. I've tried other painters tape (the blue 3M and Duck brand) but they don't have the sticking power of this green tape.
Most of the teachers at our school use this tape exclusively now to hang things on their wall and showcase student work in the hall. In fact, I ran out and had to run to the store today after school to get some because my left over blue tape was just not working (hence the post).
If you are looking for a good tape for school walls I would highly recommend it (cost with tax is $9.30 a roll).