Hm, this reminds me of a Supreme Court Case we learned about in Government this year (Yes, I DO pay attention on occasion), I believe it was something vs. TLO, and it involved a girl being caught smoking marijuana in the bathroom at her school, and she was sent to the assistant principles office - where he proceeded to empty her bag, where he found a note card with everyone who owed money to the girl, joints, and more Marijuana.
HOWEVER, I believe they need probable cause to search through your PERSONAL belongings, we have random police dogs to come in and search our lockers and sniff our stuff as well, but they NEVER go through purses and bags and stuff, and I've never even heard of them searching cars, thats a total violation of your Civil Rights - like Lexi said; unless they have a warrent - they aren't aloud to search car's without consequences, at least thats what it is around here.
So I DO believe Meghan did the right thing, Lockers and stuff that the school owns is fine, but going through your personal belongings is out of the question. At my old school we had frequent lockers checks and such - since my previous school had a history of bad students and contraband; IE Guns, drugs, knives etc. But I don't think anyone ever looked through your bags unless they had a reason to believe you were hiding something.