ONe of my student's favorite programs is Comic Life. If you want to do great comics without a drain on the bandwidth at your school, this is the software you should consider.
Web-based Comics. BitStrips for Schools places it on a private virtual network where teachers can moderate content and review all characters and comics. Students can flag inappropriate comments or bullying, which is then instantly removed pending teacher review. There is a nominal fee of $9.95 per month for up to 40 students.