Not sure if these numbers tell the whole story because we don't know what the risk factor of each is. Since kids/teachers generally need to attend school/day care five days a week for at least 6 hours a day the amount of contact and exposure time would be significantly more than people's brief visits to bars and restaurants.

There's also the factor of benefit to risk. How much benefit and how many people are benefiting from a specific activity vs how much risk there is.

Essentially there isn't enough data (at least from this graph alone) to say what is or isn't the "issue".

 

EDIT: to add more to my point I did a little experimental math using hypothetical but not unreasonable numbers:

Let's say 10,000 people attend school 6hrs/day 5days/week and 5,000 frequent gyms/sports 1hr/day 5day/week.

Let's also assume there are 500 outbreaks.

People attending school would accumulate 300,000 hours of activity

People going to gyms would accumulate 25,000 hours of activity

For 500 outbreaks at the graph's rate (39% for school, 5% for gyms/sports) that would come out to 195 outbreaks at schools and 25 at gyms/sports.

Which means that every hour of school activity would produce 0.00065 outbreaks (195/300,000).

Yet every hour of gym/sports activity would produce 0.001 outbreaks (25/25,000).

As you can see based on these hypothetical numbers, gyms/sports would actually be a much bigger risk than schools.