I am wondering how to determine what size of school sweatpants to order off of my schools orderform ?

My schools clothing orderform just came out and i decided i am ordering a pair of school sweatpants. They are really cute, have STA written horizontally down the left leg and "Thunder" across the bum and are grey. Only problem is I have no clue what size to order. I am in the 10th grade and am average size. I wear a size 7/8 in jeans. These are uni pants so the sizes are generic I'd ume. The sizes are S (small), M (medium), (large), XL(Extra large). I ordered a pair last year in large and they were much to big for me also. Thanks in advance for your help.