I don't really know if you can find that statistic. It would be interesting.

I have 2 kids that started private in K - they are in 3rd and 5th now - with 50 classmates each.
Both left their school for a different private (after consulting with the local public school special education teacher only to find out we did not qualify for IEPs)
I have 2 friends each had 2 kids so that is 4 that left due to divorce and financial issues.
One left for pure financial reasons and went to public.
One left to move and I believe ended up in public in the midwest.
Six (2 came in 1st grade) left simply because it was a bad fit and went to local public. - 2 had siblings that stayed
Once left for an all boys private.
Three left and went to another private school for various reasons.

So that is 12 that went to public out of 100...
and 6 that switched privates.

I assume I don't know all 100 kids so there may be more.

My 2 kids' current private has 30/grade and lose about 1 or 2 each year so far - some to public, some to homeschool, some to other privates.

What percentage of children starting private school in preK, K (elementary school) do not reach Grade 12 because they leave for area public schools?

What is the average private school life span in years of children starting private school at time 0 (elementary school -- preK, K, grade 1) before heading to area public schools?