Manya Whitaker, PhD, Developmental/Educational Psychologist; Assistant Professor of Education; Educational Consultant
It sounds like you are qualifying heterogeneous with respect to age and ability so my answer will respond in kind.
What we know from decades of research is that all students across all ages K-12 benefit from multi-age and multi-ability classrooms. This is because students, especially secondary students, are very likely to learn from their peers. In such collaborative learning environments it is helpful to have people who are older and those who are younger because it offers you the opportunity to have a social/academic role model and to be a role model.
For example, mixed ability classrooms are essential because according to Vygotskian educational theory, they make it easier to find students' Zone of Proximal Development. Every student has a unique base level understanding of something and a unique potential achievement level. With mixed ability classrooms, there is more likely to be a more knowledgeable person (even a classmate) who can help you. In turn, every student is likely to be the more knowledgeable person in something and they can help a classmate.
We actually do the opposite of this in our current single-age (although very few classrooms are actually comprised of 100% of the same age) through ability grouping. Teachers do this because of time constraints. No teacher can work with each student individually so instead, we group students who perform similarly together and hope they can help each other. This actually is the opposite of educational and psychological theory.
If you are looking for a school with multi-age and mixed ability classrooms, look for a Montessori school. Some Waldorf schools also have multi-age classrooms, but it is not required of a Waldorf education as it is for Montessori.