Everyone in my company who works in a safety critical role is subject to random D&A screenings. No drinking while on duty (even in lunch hours) and if drugs are found in your system, it's instant dismissal. I think this is completely understandable for such a situation.
With regard to introducing such tests in other work environments which may not be safety critical, I would fully support a no alcohol policy in work time (ie, lunch time drinks) as I feel it slows responses and therefore means people are not at their most productive.
Drugs (out of hours) is a bit more complicated as it's about what an individual does in their own time but it should be remembered that the practice of taking drugs is illegal - as such, it's possible that if someone gets into trouble in their personal life, it could have an impact on their employer; (the way that Burberry, Rimmel and so on cut Kate Moss (temporarily) as soon as her drug taking became public, due to the impact on them).
As long as potential employees are fully aware of the company's stance on the matter prior to employment, then the individual can choose whether such a clause effects their life style.
If a company were to implement such a policy, any employees who already worked for them would be protected if they didn't agree with the stance as their existing employment contracts would not have this clause in them.
With regard to schools, at such a young age, any deterent is a good thing (in the same way that kids get detention to deter them from skipping classes).
At university, the students are adults and as such make their own choices. However, if a policy is implemented by a university, in the same way as an employer, as long as the students are aware, they can decide whether that's a university they want to go to.
