Don't worry -- this is SO NORMAL. Between 12 and 24 months tends to be the peak of separation anxiety. We're just coming out of it with my 2-yr-old, and I know how frustrated you can feel sometimes. (For heaven's sake, I just want to pee in peace!) I've found that there isn't much I can do about it if I'm at home -- he's going to want me if I'm there.
As for leaving her while you go back to school, it might be tough at first, but she will eventually adjust if you've found a good place for her. We were lucky to have a sitter part-time that my son already knew by the time I went back to work. If you are able to leave her with someone she already knows, that might help, but it might not be an option for you. If you can swing it financially, you might try to get her settled with the sitter or daycare for a couple of weeks before you have to go back. That way, you can be available if she's completely inconsolable the first day or two, and you'll both have time to adjust to the routine before your own work gets tough. If she has a blanket or other lovey, make sure she's got it. It is really hard at first, I know, but I've made it through this point with 2 kids now, so I promise it does get easier!