<human>: This sounds extremely juvenile, but I've found myself becoming uncomfortable with my friends recently. I was brought up in a conservative home, with strict parents. I had it drilled into me, from an early age, that anything illegal was completely and utterly out of bounds.

My friends have recently become friendly with this pothead who dropped out of high school. I'm not friendly with him at all, in fact I've never even spoke to him. When I first heard they'd smoked weed when hanging out with him, I wasn't too worried. Just because they did, doesn't mean I had to.

Yesterday, however, I went to a party, where one friend, lets call her A, and another friend, S, also came too. A drove me there and was my lift back, so I had to stick with her for the whole party. Turns out S had arranged to get some weed from the pothead, O, who had gone off to get it from his dealer (that's what they told me anyway). I was a bit anxious about this. In the end, S and A walked back to his house at 11:30 pm. I had to go with them because there was no other way of me getting back home and the party was over.

There were a couple of other people at the house, completely pissed and smoking weed. This is where I felt uncomfortable. I didn't feel like I could say anything to A because she was driving me back home. An hour later, O eventually came back, high and pissed, and gave S shit loads of weed. None was offered to me, but I was extremely uncomfortable. S rolled up a joint on the way back home but didn't smoke it. 

I can't tell my parents because if I do, they will, for sure, tell S's parents and in the end I'll get all my friends in trouble. But I don't want anything to do with these people anymore. I've been drifting away from my friends for a while, towards another group of people who I generally prefer as company.

How should I let them know I don't want to be friends with them anymore? Should I slowly stop hanging out with them, or just tell them up front?

Could you distill the essence of the text into 1-3 sentences?
<bot>: Went to a party, ended up going to a pothead's house, friend got loads of weed. Friends regularly get high after experimenting with weed, I don't want to hang out with them anymore.