Basically, the standard is that there needs to be a reasonable expectation that the locker could be holding some sort of contraband. Examples:
Reasonable: You possess the kind of paper used to roll joints OR there's a plainly visible beer bottle in your backpack.
Reasonable: You smell of marijuana or alcohol.
Reasonable: A parent expresses concern that you might be selling drugs out of your locker.
Unreasonable: You listen to Tenacious D, who have spoken about legalizing marijuana. Since you approve of their music, you must also smoke marijuana, and might have some in your locker.
Unreasonable: You smoke cigarettes and are young, so thus you must also smoke marijuana.
Unreasonable: The Principal gets a note saying that you sell drugs from your locker, but the note is completely anonymous. (The source has no credibility, esp. since it likely came from a student, inherently unreliable to begin with.)
There's another California case on the matter as well, Gordon v. Santa Ana Unified School District. The Principal ordered Gordon to turn out his pockets, based on previous misbehavior, some old info, and the fact that the student talked on his phone a whole lot more than average. They found marijuana in his pocket. Gordon was expelled and was going to face criminal proceedings; however, the search lacked reasonable suspicion, and thus was inadmissible in a court of law. (NB: To be admissible, the search has to be legal for whoever conducted it; if there had been reasonable suspicion but no probable cause, the contraband would still come in because the search was made by an unsolicited school official, and not the police.) The irony is that the Supreme Court couldn't overturn the expulsion, since they have no jurisdiction over decisions made by the school board; what the school board considers legal evidence collection is up to them. I do have to wonder what would happen in a civil court on that matter.
The lock would be an example of reasonable suspicion. Refusing to give the combination to school officials makes it seem like there's something to hide in there. Note that this standard fails probable cause (if it didn't, police could enter any locked house on that basis), but it's not entirely unreasonable.