Optional by who? The student or the parents? If it was optional to the parents why in the world would they allow their child to go a class called "The New Gay Teenager"?
The web site seemed to indicate that there are several optional seminars. I'm sure it is the students who choose.
The seminar wasn't scheduled before the resident "school" began, wasn't revealed to parents until afterward, wasn't clearly identified as optional, and had no competing options.
Circumstancial evidence, but compelling.