Actually when you attend school, the school basically becomes your parents. If we are talking about High School. They can search you, your locker, backpack, and or car if they want to. You basically have no rights while you are at school. There are several supreme court cases that have sided with the school.
However, if I were you Thanatos, just say you were around people that were smoking or you ate popeyseeds. These can also make you fail a drug test. Also do you know if they were testing for pot?
The exact reason I dropped out of highschool.
"When the people find that they can vote themselves money, that will herald the end of the republic."- Benjamin Franklin