Used kids clothing sale! Your target audience has the clothing and likely the need given the way you describe the school, both the sellers and buyers go away getting something real for the money that is spent and you get a percentage of the take. Our school has been doing this for years and when my wife helped run the thing 10 years ago they made over $60,000 in two days. At this point the school is know for the sale and people wait for it as they fill the gym with clothing and the selection is second to none in the area. It will take years to get to this point but I'd bet you could do very well even the 1st time around. PM me if you want to learn more.
Bob