Apparently my SIL exaggerated that it was for the teachers, its actually going to be raffled off to raise money for the school.
That's not an exaggeration, it's completely false.
But I was concerned about how technically police could confiscate it which implies a breach of law
The law around minors and alcohol is more or less completely focussed on purchasing. Under 18s cannot purchase alcohol in shops and supermarkets.
If you're five or over and under 18, there is no legal restriction on you drinking alcohol at home or on other private premises.
A person of 16 or over can have beer, cider or wine in a pub or restaurant to consume with a meal at a table. They must be accompanied by a person over 18 who must purchase the drinks.
Persons under 18 may not possess or drink alcohol in a public place.
If police suspect a young person of possessing alcohol (or something they believe to be alcohol) in a public place other than licensed premises, they
can confiscate it. They do not randomly stop youngsters to see if they have any booze; this law is really intended for when groups of kids hang around in places such as malls drinking from cans and bottles and misbehaving. A 17 year old can take alcohol to a e.g. a party on private property without fear of getting in trouble.
I know Northern Ireland is stricter, and in Scotland the 16 and 17 year olds having beer wine or cider with a meal can order it themselves, they don't need an over-18 present.
I was struck about the relaxed attitude towards alcohol and youngsters when I first arrived, but now I see many US attitudes as tending towards being puritanical. I personally think a balance about halfway between the two might be a good thing. My kids had wine from 11 with meals, watered 50-50 till they were around 14, and they have become responsible drinkers with quite refined tastes, I am pleased to say.