As far as I'm concerned, Mandy and her grades have been a topic of fascination for me. In Dental? More Like Mental, her mother Phoebe cuts off her credit card until her grades are brought back up.
Then, in Alex Gets Schooled from Season 4, it turns out that Mandy was the person who got all Fs (since there had been a mix-up at school, and the report card belonging to Alex actually had As and Bs). Phoebe then grounds her for the rest of the year. For Mandy, that means no going shopping at the Beverly Hills Mall, no picking out new clothes to wear, and no dates. Instead, she has to improve her bad grade point average. Too bad for Mandy. She had to learn the hard way that actions have consequences, and disobeying the rules means being punished or having privileges taken away, too.