Program there using is called lanschool. Currently, there are a few methods to block it.
One, pull the own cable out. No net, but local minecraft will work.
Two, duelboot using hirens, go into minixp, and delete the hole lanschool folder.
Three, it you have cmd access, open it, and enter "tskill student.exe /a" and hit enter. If it errors, use method two.
Packet sniffing for grades will not work. The encryption on the standard school grading and database programs will make it pointless to try. If a single packet's wrong, it just errors, and skips it.
Hope that helps.
Btw, nice forums, might come here more often.