I think focusing just on resources misses part of it.

Upper class parents know how to raise children to present as higher class because they have the benefit of having been raised and lived in that class.

Being able to spend time on my kids helped, but they also entered school at a high level in math and reading and with the diction of a higher class because I knew how to teach this to them.

Some of the knowledge of how to succeed in education and develop children's minds is unevenly distributed, and it's not something easily fixed by just committing resources. (Though committing resources surely helps).