I can't speak about the student scene, but among people my age (I'm in my 40s), many people seem to be suspicious of other people who don't drink at all, unless they have a clear reason - a health problem, driving, pregnant, etc.
The girlfriend of one of DH's friends has a story about an American man who came over on a business trip. He went out to the pub with her and some other people after work, and he tried to limit his alcohol intake by buying drinks and not finishing them, and just leaving the leftover drinks around the pub, so it looked like he was drinking more than he was.
My friend thought there was something wrong with this behaviour and it made her dislike this man. She that she and another woman kept picking up his leftover drinks and bringing them to him, saying "You didn't finish your drink, love." I think that pretending to drink more alchohol than you are is seen as dishonest.
I think there is the idea that if you choose not to drink with anyone else, you are trying to distance yourself from other people.
I think that's where the round system comes in; you're all drinking as part of a group and everyone is drinking at the same time and everyone knows what you're having.
The people I drink with know that I don’t handle alcohol very well, and that I can only handle a couple of cocktails a night. I still do the rounds system, I just drink very slowly, timing my drinks, so that when it’s time for someone else to buy a round, there is a sufficient amount left in my glass that they aren’t going to question why I don’t want another one.
When it’s my turn to buy a round, I buy drinks for the other people in the group even if my own glass is still half full.
ETA: I just remembered. At my old job, we had a guy who came to work for my team temporarily, and the manager was describing him to us before we met him. One of the thing she mentioned, besides things like where he was from, his previous work experience, etc., was that “He doesn’t drink”, as if that was a big deal that everyone should know.
At our first social outing after he started, she was asking him why he didn’t drink – Is it your religion? Do you have a health problem? And he said that he just didn’t like drinking. She didn’t seem to think her questions were inappropriate.
If I didn’t want to drink at all, and I wanted a soft drink instead of alcohol, I would just say that I was very tired or have to get early or something like that. Personally, I wouldn’t pretend I was drinking alcohol when I wasn’t.