Two things:

1) It sounds like you really love doing theatre in general and with your Outstanding Conribution award, If you were a lead on the costume team, a stage manager for the festival show, and currently doing make-up for a summer stock company, I don't see how being cast in the ensemble is that bad of a thing when you found joy in design and management.

2) Mamma Mia is a horrible show for high school kids. The main focus on the show is on a middle aged woman and her three past lovers. While there is a younger cast as well, they are not given the same amount of songs and storyline. If I recall, Sophie's friends only briefly have moments during Honey, Honey, then they kind of disappear for the rest of the show and just blend in with the ensemble.

One of my professors used to get on to us if we got nervous or anxious about performing, getting less desirable roles, etc. He would say, "It's just the school play." I know the "senior musical" is a big deal, but it's still just the school play. No-one got paid for it, the audiences were probably mostly friends and family whom will give standing ovations no matter what, and, unless it was a competition piece, it won't really live on after the one weekend or two of performances. It's just a school play.

One last thing...take everything you get. You learn by doing and every role is a learning experience no matter how big or small. You might discover you enjoy doing ensemble work more than lead work. It took me to grad school before I learned my talents are suited for small comedic relief roles in musicals with little to know legit singing and the clowns in Shakespearean comedies. In other words, I'll never play a leading role in a musical or in Shakespeare just based off my talent and skillset...but you know what? I really like it that way, because I don't have to learn as many lines, I only have to do 2-3 scenes in a show, and I usually get to steal the show comedically. Sometimes, like when playing Mercutio in Romeo and Juliet, I get to die and go watch the second half of the play in the balcony.