"he had to stay after school and also drop swimming in order to practice running"
While this is a bit exagerated in comparison to what I've seen in the K-12 setting, it does remind me of the expectation that everyone be at the same level at the same time point in their education. I've worked with home-schooled students who were 'diverse' in their subject standings on admission to high school. They were able to move up in their own time but didn't respond well to excessive concern over their inital standing.