It's up to the Steering Committee who gets promoted, unless a group self-promotes. The top three performers at the World Championship in the Open and A classes are automatically promoted each year. Some groups will get bumped up during their seasons, if the Steering Committee thinks that they are competing in the wrong class. The Steering Committee reviews videos of the top few performances at each contest.
Avon gave the best performance in Scholastic World. I think Trumbull might have outperformed Carmel as well in Finals, although I much prefer Carmel's show to both. It has that spark of ingenuity the others lack. Also, it's nice to finally see everyone's faces and the details of the costumes, etc.
Thanks for the clarification!