Even if not one of the other 8 kids touched a drop, which sounds highly unlikely, the OP reports only 4 bottles of weak lager & a drop of prosecco gone if I recall. That wouldn't have made me puke at 15 (that usually took a bottle of QC sherry...).
Someone has smuggled something considerably harder in. Probably, but not necessarily, puking girl.
& this is EXACTLY what is going to happen if you allow teenagers to drink.
If it were me (few years off with my own kids, but teach teenagers), I'd probably have a few beers on offer.
BUT it would be a clearly stated house rule that no visiting kids were offered a drink, unless I knew the parents personally & knew they were on the same page as me. No hard stuff & no smuggled booze - if I don't know it's in the house & being consumed, not ok.
Known guest openly bringing their own couple of beers ok, vodka in a bag - no.
Any breaches eg. Puking random, & the parties would be suspended, & I'd be tracking down the other parents to tell them: 'Your child was at my house last night, I'm sorry to say they were drinking, & I don't know if you allow this.'
If they wanted to be furious with me, fair enough, although I'd point out that I did have house rules & these had been broken by the kids.
Also, in these circumstances (not knowing the parents or how to contact them) not just the alcohol but also the girl would have had to be smuggled in. Because I would BE IN THE HOUSE & would have been aware my dd had a guest I didn't know, so would have been in to chat to them & be Buzz Kill Mum by insisting there was no drinking going on as I didn't know if other parents would allow.
Sorry OP & I bet you've had a grim night of it - but dick move by you there.