If I'm a parent who is typically interested and motivated, am I committing a sin against social justice if I am interested and motivated in my children's education and work to provide opportunities for them to learn?
You're not sinning, you're just reacting to incentives.
This isn't a moral condemnation of you, it's just the fact-based explanation for why schools that you have to apply to produce better academic outcomes than default public schooling. If those public schools could just drop the cohort of students whose parents couldn't be arsed to apply to a special school, their quality would also go up.
Why? Because "cherry-picking" is bad and the best things for children are good?
It's, of course, exactly cherry-picking to find the children with the most interested and motivated parents, then filter them down to the ones that you like best. Fuck the kids with bad or no parents.