Should schools have the right to search student's purses, pockets, and bookbags?   stance: yes:
I see that there's actually no arguments, so I'm just going to clarify a few things for the 'Yes' argument.
Even though Principals do obtain the right to search (for drug tests, even making the student take off his/her socks if the dog sniffs something on that student), they only retain that right in the event of reported drugs or crime, or in the case that sniffer dogs checks are enacted and the dog smells something.
There are restrictions - a principal can't just demand to search a student's possessions on whim - outlined in the Criminal Code. YCJA and Canadian Charter of Rights (as well as parallel US laws).
Thus, the right keeps the entire student body safe and away from harmful substances.