It is REALLY a bad idea and yes I would say it has absolutely no standing within the school and I'm sure it has no legal standing. There may be something the PTO does that is required to volunteer (like a background check or confidentiality training) but I would head straight to the principal and or president and let them know that the policy is offputting and unnecessary.
I have just moved to a new school system. A flyer has come home stating, "In order to volunteer to be a room helper, a Dad's Club member or come to classroom parties and other classroom functions, you must be a PTO member." Is this lawful? I have no problem with becoming a member, but restricting parents from the classroom does not seem right to me. Also, the wording does not seem to encourage parent involvement - it's more like, "join our PTO or else!"
