I'd like to think this never happens, but I am sure it does, on occasion. I don't think it happens everywhere all the time though.
My eldest is 17. We've discussed what goes on at parties, how they sometimes get out of hand. We've talked about how she can ask me to come and get her, no questions asked any time.
I've also overheard and seen my DDs discussing things with friends when they think I'm not around, so I know it's not all sweetness and light.
Certainly for my DDs (the older 3, my youngest is only in Year 6!) it seems that alcohol is a huge feature at every party. My eldest has come home drunk on several occasions, and I'm not naive enough to think when she stays at her friends houses she stays sober. DD2 (15) has admitted to drinking at parties but has never been obviously drunk when I've collected her. DD3 (13) denies having drunk alcohol at parties, but I wouldn't be surprised if she has. So I would say alcohol at parties from Year 9 onwards seems to be a constant.
My DDs have discussed girls and boys behaving promiscuously with me. It seems that it is usually one or two girls who have usually had too much to drink behaving in this way, with a few boys. The boys don't seem to get as much stick for this, but then my DDs say when the boys get drunk they tend to get loud and then vomit, whereas the girls get flirty then vomit. I've discussed with them all that if a girl is very drunk and boys take advantage, that is wrong.
I've heard them discussing with friends how 'such and such was a right slut' and other similar things. I've had the talk that a girl being promiscuous is no worse than a boy, and vice versa, but this does not seem to be the message amongst peers. DD1 has said it's because the girls actively 'flirt' whereas the boys just take what they can get. She said if a boy 'flirted' in this way, most girls would bat them away if they were drunk, but boys seem happier to go along with it.
I've never heard them discuss quite such graphic sexual behaviour, it mainly seems to be kissing, and couples in relationships taking it further. There was outrage when a girl in DD1s year gave a boy who wasn't her boyfriend a blow job at a party, and this was a hotly discussed topic so I can only hope they means it was a one off.
I have tried to instil in my daughters a sense of self worth, in the hopes this will protect them from this sort of thing. We have discussed getting too drunk, and we have discussed putting yourself in a risky position. It's bloody impossible though, and I'd hate for my DDs to behave like this. It worries me that they do drink, I know DD1 has drunk way too much before and been in a state where she would be vulnerable. They do dress older and I hate the trend for short and tight dresses. I console myself that they go to parties at people's houses rather than in a public place so are mixing with peers. I have told them they can call if they are in trouble or have drunk too much and I will come, no punishment. I try to talk through all the consequences but I think at the end of the day you just have to hope for the best.