My stance is simple. It is not the job of any school or any government to regulate what I as a right-minded adult put into my body as long as I am not hurting anyone. Random drug tests break down any semblance of a relationship of trust between a school's administration and its students, as it automatically assumes that all students are criminals. Apart from being offensive on a very basic level, drug testing (especially in public schools) it expensive and a drain on taxpayers.
The school's job is to provide the tools for a successful education, not force them onto you. If you choose to smoke all day and learn nothing, then Darwinism will deal with you, but it is ridiculous to think that it's ethically justifiable to punish the straight-A student who goes home and smokes a bowl to decompress after a stressful day of hard work.