You can meet one person from another household for exercise.
But only with SDing, so they should be 2m apart.
The idea is to restrict the amount of time people are moving outside of their households.
Mixing in school is necessary - eduction is necessary and SDing isn't possible, so it's a necessary evil.
Mixing outside of school isn't necessary. Playing with your mate for an extra hour after school isn't necessary or essential. It just increases the time spent in close contact with one another, and if playing then more likely closer together, shouting to one another, face to face time increased, etc.
With younger children it also means more chance of the parents standing together and chatting, often closer than 2m and often face to face and prolonged durations.
Your child's teaching and school staff have no choice to put themselves in a risky environment with no masks, no SDing etc so it's important that you and your children so everything you can to reduce your unnecessary contacts to help keep them safe too.