YANBU
Yes, teenagers will try to experiment with drink etc, but that doesn't mean all adults who are responsible for them and love them should just accept they can do anything they want to without any consequences.
Binge drinking is not something adults should condone it (yes, we have all done it, some still do) because it can be damaging to health and in a minority of cases can set up patterns of behaviour. That, and drinking can cause teenagers to end up in situations that put them in vulnerable and/or dangerous situations (as it can with adults, I suppose). Finally, what these pupils are doing is against the rules- they will know they are doing wrong and doing so on school property. Teenagers need to learn actions have consequences.
I'm a doctor- I have seen the effects of alcohol excess first hand, and they certainly aren't limited to a few embarrassing stories and a bad headache the next day. And I'm not just talking alcoholic liver disease. I like a drink occasionally, and drank excessively as a student, so I'm not naive enough to think that people won't do it. However, I wouldn't be happy to merrily condone alcohol excess in school age children.
As a junior doctor in A&E I saw so many injuries/broken bones as a result, I also treated a young woman who was found passed out in the street- she had been assaulted but had been so drunk she couldn't was unable to recall enough details for a full investigation (I am NOT saying she deserved to be assaulted because she was drunk- just that she was in a vulnerable state due to drink and then couldn't get justice because she couldn't remember details).
One of the most harrowing I experienced was the young man who'd got drunk, fell over and hit his head- suffering a massive subdural haemorrhage as a result. Nobody noticed a deterioration in his condition- the friends he was with were drunk too and had initially put his symptoms down to alcohol- they thought he'd passed out when he lapsed into unconsciousness. He ended up in ICU and severely brain injured.