I have recently set up my own group where people can request items as there are a lot of selling groups in my area already.
I have chosen to set mine as a closed group because that way when someone posts an add their friends who aren't members of the group are unable to see what they are advertising. I prefer it to be this way as I wouldn't want everyone on my friends list knowing what I was trying to sell/buy.
If you advertise your group well enough through the school at the beginning or just invite mums that you are already friends with it shouldn't take long before you have plenty of members.
I have found the admin side easy so far, all I have had to do is approve people's requests to join which makes it easier to weed out those that want to sell odd unrelated items.
Good luck with your group it sounds as though it would be useful
We are thinking of setting up a facebook page for our school parents to enable them to buy and sell items without paying ebay fees. Does anyone know the best way to do this? Is it best to set up a closed group or just a standard page? Any advice gratefully received!