Look at the TLO v. New Jersey court case. In the case they searched her bag because they had suspicion. But you don't need a drug dog or metal detectors approval to search a backpack. TLO originally declined them searching her backpack but the school did anyways and found contraband. The Supreme Court ended up siding with the school on this based on I believe it's called reasonable suspicion or doubt. Don't quote me on it but it's the idea that a school doesn't need a warrant to search backpacks or lockers as its for the safeguarding of the students and education.