"Hot" no longer means "pwoar" for teens though, OP. I don't think this is a sexual advance on her part, just a kind compliment.
I agree that the younger ones use it as just something to say to each other, and comment on pics. But the 18yo - really? Unless she's very emotionally immature, it's not a normal thing to do. I'm not much older than her, I was at school not that long ago and writing 'hot' under a lad's photo would have been really cringey and embarrassing!
Maybe it is all a joke, saying that. I don't know though, as I said before my school friend was definitely targeted by a very emotionally immature 18yo girl when we were in year 9.