Ryan, you are in PA like me. I would say walk through the mall near you, or a Walmart. Look at the shape of the girls in this age range. Me? I would say you better stock some Larges in your line up.
If you only get to choose between two sizes, I'd go Medium and Large. A smaller girl can always wear the Medium, but you cannot go the other way around and get a larger girl into the Medium. I think you stand a better chance clothing more of your target market with Larges and Medium than Mediums and Smalls.
I truly think it's worth a walk through the mall one night to judge the general size of your target market, and who may show up to the concert. Happy selling.
Girls Sizes 
