Not trans movie per se, but I like
Just One of the Girls.
Guy pretends he is a girl because some other guy bullies him as a guy, but does not bully him as a girl. Unfortunately for him, the bully is attracted to the pretend girl. The guy who pretends to be a girl at school and elsewhere, even cheerleading, also likes this particular girl at his high school.
Starring Corey Haim.