I teach the third required intro course in a CS department. One of my homework assignments asks students to speed up code they have written for a previous assignment. Fact
Darcs is an excellent DVCS, especially for smaller projects such as ones for CS classes. I wish I was introduced to Darcs or Git in college, and I also commend you for introducing it to your students.
I use Git on a daily basis. It's a very robust DVCS, but maybe a bit of an overkill for smaller projects.
Take your pick, either of those version control systems are really good.