The idea is, you have to give school officials broad deference so that they'll be willing to conduct strip-searches and whatnot when it's important for them to do so. if you open them up to liability, then they may act too timidly when a student's welfare is at risk for fear of being sued. Say they suspect a student has heroin on their person and is dealing, because another student said so. If they're wrong, even if they had a decent reason to believe there was heroin, they'll have to defend themselves in court, in front of a jury that may or may not be sympathetic depending on the kid. That's a terrible position for a teacher to be in.
I don't really buy it, to be honest. It might be a Canadian thing that I share with NEB (I cannot imagine a Canadian employer other than the police or military ever doing employee drug tests, for instance, though I'm told they're pretty common in the states), but I have trouble with drug hysteria. They can be dangerous, but they're not an IMMEDIATE THREAT! THAT MUST BE DEALT WITH!! and to hell with checks and balances and common sense.
Clear reason to believe that a student might be carrying a weapon? Widespread violence? Search 'em, put in metal detectors (terrible though that is), do what you can to protect students. Luckily enough, most weapons can't really be concealed in a 13-year-old's underwear.
But drugs? I don't see why a school administrator should have more or broader authority than a police officer with these things. If an administrator suspects that a student is dealing heroin, he or she should call the police. If there is a case, warrants should be issued, homes searched, whatever. But there're very good reasons why law enforcement can't perform discretionary body searches, and they apply every bit as much to teachers. And we can see manifest demonstration of at least one or two of them in this case.