What does Melinda feel should be illegal in Speak? Why?
The answer to your question can be found in chapter 1 of Speak otherwise entitled "First Marking Period" and in the section deemed "Fizz Ed." The title of the section actually contains the answer to your question. The school subject that Melinda feels should be illegal is gym class. Why? Humiliation. Melinda says it this way:
Gym should be illegal. It is humiliating. (18)
Melinda goes further and explains exactly WHY gym is humiliating: having to change clothes in front of her other classmates and, worse, seeing other athletic girls who don't CARE about changing clothes in front of their other classmates.
Nicole doesn't mind changing her clothes in public. She even changes bras, wearing one sports bra to regular class and another to gym class. Never blushes or turns around to hide herself, just changes her clothes. Must be a jock thing. If you're that strong, you don't care if people make comments about your boobs or rear end. (18)
It doesn't help that Nicole is one of Melinda's ex-friends who turned on her when she called the police on the night of the party.