Having experienced the level of stupidity,whininess and ugliness parents are capable of, thanks to the local baseball leauges, I can see why parents would be barred.

Our school allows parents to be present during classes.They are all well behaved, and have no interaction with the kids during class.

I believe if the parents are well behaved, the rules for their presence are laid down and enforced from the get go, it is ok for them to be present.