SCAB stands for School Clean-up And Beautification duty, typically assigned to students as punishment for some wrongdoing. The acronym was invented well after the term was already in wide usage. It was originally labeled Scab duty simply because it involved picking up garbage. In the 1980s (when this movie was set by the way), anyone who was caught rummaging around in the garbage for cans or bottles (to recycle), or food (to eat) was labeled a scab. In fact it was even used as a verb, as in "The school bully threw my bag in the garbage, and I had to scab it out again". So that's where the term comes from, purely due to the association with garbage.