[Moshineko]

Freedom comes with responsibility. You are responsible to use your freedom respectfully, and for aims that aren't inherently harmful, or overtly rude, to the public in general.

And if we want to get into a whole big discussion about Freedom, remember that the men who wrote the Declaration of Independence defined 'people' as white, land-owning men. Even today it's not applicable to children and adolescents, in the same way flamethrowers are not accessable to (hopefully most) children and adolescents. It's too much trouble to see if you're ready for it on a case-by-case basis, so you have to wait until a majority of people are, frankly, less apt to act like complete idiots. (Which should really put the age of majority up around 35, but hell, I couldn't imagine living with any children I might have for that long)

And what happens when the school turns its eyes to the public affection, and a student turns up pregnant? Rational people will declare this is the fault of the students and their parents. Rational people are very rare in cases where daughters get pregnant. It's better to not allow it than have our already paperwork taxed schools have to hire another secretary to deal with legal proceedings.

Aside from all that, I haven't actually seen a compelling reason to have schools accept that manner of behaviour. If you're teaching children socialization, part of that is that we don't generally make out in public. Hell, until about 40 years ago, we didn't even hold hands in public, and showing ankle was risque. Basically, any argument I've seen here so far in favor of schools allowing public displays of affection goes along the lines of "that's not fair!"
I've seen decent arguments against the reasons schools give for disallowing the behavior, but that's rather like sinking a sieve. Everything else is a selfish grasping for a right that's not even, at that age, truly a right at all.