I don't really agree with the above @LifeIsShambolic She is an adult and not far legally from being an adult.
I remember telling my Mum I was staying at my boyfriends. Oh no you're not! Well - yes I am because I'm 18 and you can't control me any more.
I'd actually just let her get on with it to an extent. She needs to find her own path. For what it's worth, I'm autistic but wasn't diagnosed until my 4Os and kicked off plenty at your daughter's age. Including dropping out of Uni at 1 month.
It's her life to explore how chooses. She's probably having fun for the first time and college may well have dropped off the radar.
Are her friends from college? Best thing you can do is explain - not tell - and not a lecture... But if she fails her college courses and friends pass, then her friendship group will move on.
I'd just give her time. I know it's a worry, I've a 26 year old and a 15 year old, blummin heck I'm a grandma. I tried to put foot down with my son so many times it destroyed our relationship. We're best of friends now, but there were a lot of empty years.
I should have said that's fine, I worry about you. I know you don't think I need to, but I do. Please let me when you are going out. Please let me know you are safe and please let me know by 11pm if you are coming home or not so that I can sleep. My phone will be on always if you need me.
4 simple asks with no judgement.
You can't force her to do anything else, so start there and respect her so she respects you.
It's hard work isn't it!!