As a grad student, I can tell you that (at my university at least) we do a decent amount of drinking with our professors around finals time. We don't take finals (usually), but have end-of-semester papers, so during our allotted time to take a final, we usually go to a local place and drink and have pizza or something. It's really fun and an awesome bonding experience with people that honestly consider you 'co-workers'... just, co-workers with less expertise. ^^ Honestly, though, if you don't want to drink, then don't. The best thing about grad school (well, one of the best things) is that you're old enough that you no longer have to prove you're 'old enough'. If you don't drink alcohol, that's not childish, it's a life choice, and your fellow students and professors should respect that (and if they don't, talk with your adviser about it and it should stop). You can get virgin versions of a lot of drinks if you want to go that way; virgin daiquiris, virgin margaritas, what-have-you. If you're in the south, then sweet tea is always acceptable. But seriously, don't feel intimidated by the others cause they drink and you don't. We're not kids anymore, we're adults. And we should be treating each other like adults.

I usually ask for a club soda. People don't care what your drinking half the time, just as long as you got a glass in your hand.

This is true most of the time. I go to school for engineering so I deal a lot with beer drinking guys. I sometimes get grief for not drinking beer and some comment about my "expensive" amaretto sour tastes.
It's not that big of a deal, to be honest.

We don't do this drinking thing at my school. There is some drinking at conferences, but that's it. So this might be different depending on where you're going to school and which program you're in. Don't be worried. Grad school is not college... People have matured (hopefully) and there is no peer pressure when it comes to drinking.

I'm starting Grad School in the fall and I just graduate from undergrad. I drank during my undergrad but at the same time when I had to buckle down I didn't go crazy. Sometimes I'd enjoy a beer of glass of wine while writing a paper but when it came to studying for exams there was no way I was going to drink anything. I anticipate grad school having a much more adult feel - yeah maybe some drinks here and there but nothing too crazy. When in doubt just get some seltzer water or club soda - no one in graduate school is going to judge you for not drinking, at this point we should all be much more mature then that!

I went through my undergrad AND my graduate with people that liked to drink. I didn't drink not by choice though. both times i've been on medication that recommends you dont drink because it affects the way your body processes the liquor. I still had tons of fun. I would pick ONE drink that I treated myself to and then had soda or water the rest of the time. Your still get to hang out with everyone but you get to remember it. I actually find being the one sober person amusing because you can laugh at everyone and show them the evidence the next day lol.

Just get something else... Don't worry about that. Personally I'm usually the one encouraging others to drink with me, but, you can still hang out with everyone without actually drinking.

But like others said, people tone it down in grad school, the chances people would go party hard all night seems pretty slim.