Crucial mispelling there - I wrote "day of school" where I meant "day off school", and it makes all the difference. Corrected version following:

It's not exactly new, and it's certainly not limited to black kids. I remember, when my son was at school, hearing how about the worst insult you could have was to be described as a "boff" - means someone who was studious.

I've never heard a good explanation of where it comes from. Kids who excel in sport aren't embarrassed about it. Why the difference?

I think it maybe that sporting success is seen as a more collective thing, at least for team sports, and even in individual sports, since in a school setting individuals will normally be part of some kind of team or squad. There's a sense in which everyone benefits from the success of individuals.

On the other hand doing well in studies is more of an individual thing, and nobody else feels they benefit. I went to a Jesuit school, and the cunning Jays used to organise classes into two sides, Romans versus Carthaginians, and you scored and lost points for how you did for the side - and every now and again there'd be a day off school for the winners, and the losers had to stay in and work. Roughly the same system they used in Hogwarts.

Whether it worked, and took the stigma off academic success, I'm not sure. I think it probably did, at least to some extent. The thing is, children, and possibly boys especially, are collective in their feelings in all kinds of ways, and group loyalties matter. That's not at all a bad thing, but if it's not worked with it can have some dodgy results.