Very familiar with the prep school circut. 8th grade is still middle or lower school. Can't think of any prep day schools with hockey for 8th grade. Unless your kid is a 16 year old freshman (I've seen it happen)?
You make it sound like a 16 year old Freshman is rare. An "on time" Freshman with a birthday in the first half of the year turns 15. So any kid with a Jan - June birthday - as the majority of elite hockey players do - that repeated anywhere along the line, whether it was 1st grade or 9th (very common for boys out of a public HS to be asked to repeat academically), will turn 16.