K reading is in tiny groups at tables throughout the day. So, a child who is reading would not be sitting in a group with a child working on letter sounds. Everyone sits together in morning circle to review the plans of the day. This, of course, does involve readers of different levels.
1st-5th grade is a combination of group-read books for discussion, and individually selected reading at the appropriate reading level. Children who aren't inclined to stretch themselves will find a teacher engages them in a conversation about choosing books, and guidance is offered that balances interest and "stretch."
Research methods and techniques are introduced in 2nd and 3rd grade, and heavily emphasized in 4th and 5th. Students do individual research projects that fall under the larger topic being studied. If the integrated unit is Colonial America, each student focuses on his/her own topic, and is pushed to to challenge him/her self. The end "product" can vary by the ability of the student.
Math is Everyday Math, with extensions for those looking for extra challenges. Middle School math is using a math program developed by a Potomac School math teacher. There is not a TAG/AAP program like in public schools, but there are challenge problems at each level. A 6th or 7th grader will find challenge problems in every section/assignment of the math curriculum. Though algebra isn't taught in 6th and 7th grade, the curriculum actually includes alot of algebra. Problem solving is emphasized.
The school's literature used to say it serves average and above average students. I do not know if it still says that in its publications. There is a significantly "above average" (huge understatement

but I won't feed the test score trolls) student cohort in every class.
Sure, as a PP suggested, who the students are is part of the reason they get into the colleges they get in to. Same could be said for the students at any high school, public of private. Rather than assume these kids were accepted to highly regarded colleges "despite" Burgundy, you might try talking with those young men and women. They do not share PP's view of their Burgundy experience.