Cons of volunteering:
Any layperson who is a parent could volunteer for a job which could be done so much more effectively by a person who is qualified for it (a real teacher's aide).
I know too much about the kids, their personal lives and any learning issues they have which I am not supposed to know (privacy issues? a lot of them are our neighbors, so we meet them outside of school).
I am still surprised that while my son's Little League did a background check on me before letting me volunteer the PS just let me walk in and work with kids (just a TB test was required)! Though teachers are supposed to be vigilant and nothing bad could happen to the kids on their watch, it still struck me as odd.

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These reasons, particularly the privacy issue, are why parent volunteers are very limited in what they can do in our district. As far as I know, parents are never in a role where they directly teach or do work with students- one can shelve books, help out spotting in gymnastics during gym, etc, but they do not want parents to have intimate knowledge of the other kids' abilities or problems. Also, I consider the PTA to be very different from parent volunteering in the schools; they are separate here. The PTA here is primarily involved in fundraising (and socializing) as far as I can see, and seem to have no interest in academics at all.