Laurie, these are some of my thoughts:
It is not against the law for under 18s to consume alcohol. I therefore think that I am likely to be on dodgy ground if I try to enforce a code of conduct upon my 16-17 yr olds which the law of the land does not support, and which I myself do not adhere to.
I personally don't think it is a viable option for me to prevent my teenagers going to parties at which alcohol is likely to be available (all of them, as far as I can make out!) without segregating them from their peer group.
I think a blanket ban on attending such events is likely to lead to a breakdown in the relationship I have with them, which facilitates the calm and open discussion of things such as alcohol consumption, drugs (obviously illegal anyway - I'm on stronger ground there) and sex (also not illegal for under 18s), and in which I can state my point of view, which I hope will be taken on board, along with my expectation of sensible behaviour. That is my aim.
I would rather that, than imposing my will and having them sneak out to parties behind my back and lie about what they have been doing, which was the experience of one family I know, whose strict house rules prompted not cooperation, but duplicity. Growing up is a gradual process, and limited exposure to some of life’s dangers is essential in helping teenagers know how to deal with them.
I know that my DC wouldn't do it because I know where they are at all times
This is quite unusual, in my experience, if you are talking about teenagers 15+, hence my enquiry as to the ages of your dc.