Sleepaway Camp (1983) From Wikipedia: As the counselors and the police scour the camp, Angela meets Paul on the beach, where she tells him to undress, which Paul enthusiastically agrees to do. After finding the dead Meg and Mel, as well as the still living Ricky, Ronnie and Susie find Angela softly singing to herself and clutching a large knife and Paul's severed head in her hands. Angela is revealed to be the thought-to-be-dead Peter. Through flashbacks it is shown that after Martha gained custody of him, she decided to raise Peter as a girl, already having a son and coming to the conclusion that another boy "simply would not do."
In the others, Angela is a real girl. Here's the IMDb synopsis for "Sleepaway Camp II: Unhappy Campers" (1988): Angela Baker has undergone years of therapy, electroshock and sexual reassignment surgeries, and finally landed a job in the last place she should be working: Camp Rolling Hills. She has an old-fashioned approach as to how camp should be and an old, familiar, deadly way of making sure that those who don't follow her rules don't get to come back next summer.