Unraveling in junior tennis:

Not only do the most competitive tennis players have to start training while young, they have to train more intensely than is compatible with attending school: The Career Path to Pro Tennis Often Passes High School By

"American junior tennis has had a major change in the last five years. Aspiring pros now commonly abandon regular school for home or online educational programs. Although alternative schooling is not new to junior athletics, tennis is perhaps the only sport whose full participation requires it because of year-round competition and travel. "


Apparently the issue is that tennis players are ranked in part by the number of tournaments they do well in, so they have to play in lots of tournaments.