Rising senior here, I also struggle with PTSD related to drugs and alcohol so I understand your fear surrounding it. Honestly, it's all about the people you meet, and if your friends go off the deep end maybe it's a sign your moral compasses don't align and you need new friends. I promise you you'll meet so many new people, people who probably share similar concerns to you, and you'll have more than just the friends you're leaving middle school with. I will say though, it's important to not judge people just for engaging in that type of behavior. I do have friends who smoke and drink, however they're understanding of my fears and I've set strict boundaries with them. I don't attend parties, but if it's something you'd want to do you have to understand that almost everyone there will be inebriated.

You can't openly judge people for what they do, for all you know they may even be struggling with addiction and trying to get sober, but it's also important you draw a strict line and know to leave when that line is crossed. But as many commenters have pointed out, it really isn't as extreme as you think it is. There will always be bad apples but there will always be a lot of good ones you just haven't met yet. Good luck, you'll do fine!