My dd did Yorkshire Assembly summer school as a boarder which is reasonably priced and with exceptional teaching in both ballet and contemporary. The classes were grouped in ability based on their last ballet grade taken which meant that pretty much the children in class were all at a similar level.
Boarding is in dorms, with rooms of 4 for the under 14's and rooms of 2 for the 14 & over and the venue is the beautiful Queen Margarets school at Escrick.
My dd has been two years running and has found each year to be of great benefit, which has been backed up by very positive comments from her ballet teacher at her local dance school. It is really good value for money and a great experience for those who don't want to be away from home for too long as it is 4 days.