Most kids in school already view it as an "us versus them" environment, adding mandatory drug tests to the things kids go through in school is an act that very directly says "we don't trust you" to them and will just make them more disagreeable with the people employed at the school. An increase in tension does not make the students more receptive to learning.
Students have rights to privacy and should not be subject to any testing without a valid reason for it. If the student is attending and not causing a disturbance, or harming themselves or others, what they do outside of school is not the concern of the school. The parents are the only ones that should be allowed to test children.