Bitstrips is one of the best online Comic Strip makers I know of. If you plan to have students use this for an assignment, you will first need to give them time to play around with creating characters and all of the fun tools. It is a pretty involved Web 2.0 tool!
If you are looking for a simpler form of a comic strip maker, try this one on the Read Write Think site. Our students used this the other day and were able to create a comic strip in under 15 minutes!