Well I just see that it would become divisive - creating tension when it doesn't need to be there, embarrassing students and casting guilt. Ultimately: I see allowing certain types of groups to be a form of peer pressure.
"You're a terrible person for being 16 and pregnant and not wanting to actually birth a baby." If someone aligned with pro-life then pro-choice people might harass her and giver her a hard time for "being just a baby machine and disrespecting yourself, you're better than that"
I was pregnant in high school and I'm glad I didn't have to take anything from my peers about it. Dealing with my parents, being forced to marry a boy I didn't love, and facing my father's church was bad enough. School was the one place where my pregnancy wasn't the center of discussions.