While they were fun to use, their rarity made them that much more special in my opinion.
Perhaps if they set it up so that when you collected a book out of a locker or from another student, you could change it into say, 5-10 "kick me" signs by attending the English class it would keep that special feel and allow you to get them easier than they were really available.
As for the Volcano 4000, perhaps if the chemistry set allowed you to make just 1 per day (even when there's unlimited chemistry set uses) this would also keep that "rare" feel to them.