Busted. Strickland, the school principal, gives Marty a hard time about hanging out with Dr. Emmett Brown, a.k.a. Doc. Sounds like this Dr. Brown doesn't have the best reputation around town.
Strickland then tells Marty he's a slacker, and compares him to his slacker father… then tops it all off by telling Marty that his band doesn't have a chance of making it through the audition for the school dance, and he shouldn't waste his time. This guy has inspiration and motivation oozing out of his pores. We can see why he became an educator.