I think PDA does not belong in schools.
I think hand holding is ok, a hug, even a couple sitting next to each other with their arms around the waist and stuff. Basic displays of affection are non sexxual and non offensive, and as long as it is done outside the classroom, young adults should be allowed to have relationships.
But more serious acts, like cuddling/nuzzling, kissing, and 'petting' are inappropriate for a school setting. I think the more sexual displays need to remain extra curricular, done away from school.
There are enough hormones in schools as it is, teachers shouldn't have to regulate make out sessions.