If shit goes down in a school, parents aren't going to sue taxpayers. They are going to sue Superintendents and building administrators.
And sure, the items a person stores inside of a locker are private property, but the locker itself is not, the school building is not. There are "codes of conduct" for all kinds of locations, and one of those codes certainly is "what you can and cannot" bring into the building, and for good reason, like say, public safety.
As a parent, I want the administrators of my school to have the ability to head shit off at the pass before it ends up posing harm to my kids. Now, I also would want them to use restraint and reason and not go out of their way to cause trouble for any particular kids. Especially given that often the "good kids" can be involved in shit just as much as the "bad kids."