I think the main point is that

1. it's unfair that boys who chose to play flute got bullied for it

2. it's not good that his brother essentially took his choice from playing flute by reinforcing #1

3. #2 is actually simultaneously the way to minimize the negative effects of the authors choice of instruments

So in spite of those first two things individually being pretty shitty, they actually give you an optimal outcome (socially) in school. Ideally we get to a place where #1 stops happening, but you won't solve it by going against the grain, you'll just be a martyr for nothing.