In terms of quality that would go to Zeiss, but Sigma's definitely worth checking out. See if you can rent from somewhere, or buy from somewhere where returning it isn't a prob. Not a bad move by the teacher. And you see so many people walking about with big DSLRs these days. I'm sure most of the time they leave them at home, and would be happier with a high end compact camera. The quality and versatility are great these days.
In a lot of cases it's cheaper in comparison to the equivalent Nikon glass. The 16-35 is a nice lens. If you have a 550D, I'd also take a good look at Canon's 10-22 (equiv to 16-35 on the 550D), and the 17-55 that Tom mentioned above. Depends what focal range you mostly shoot at. They're both great quality glass. I wouldn't worry about them being EF-S if you go full frame later. Keep them in good condition and they have good resale value.
It's a good idea to look through the exif details of your old photos, and see which focal range you're mostly shooting at. And also through photos on Flickr that are similar to what you're aiming for.