Are mixed-grade classes any better or worse for learning?

Often when students are placed in a composite or multi-age class, parents of the younger children worry they won't be able to keep up. Parents of the older children worry their advanced needs won't be met. However, before deciding whether mixed-grade classes are "good" or "bad" for your child, first we have to establish what it is you want out of school - is it the best academic achievement, a focus on social-emotional development, a strong friendship group?