I do not think that requiring a B grade average to participate in after-school activities is a good idea because many students that have good skills in those after school activities may not be so good at the other classes they take.

If you limit the requirements that low then you won't get nearly enough people to have the activities and there would be no point in having them at all. If someone who was good at archery wasn't allowed into the team because of their average grade then that person wouldn't be able to go to the contest and get a chance at winning.

The students average grade in the core classes wouldn't necessarily be the same for other things that they do outside of school. Like if a student had a C average and was really good at football he wouldn't be allowed to participate which isn't too fair.

Also, if the idea is to get students to raise their grades, this isn't the best way. If they can do these kind of things outside of school then they won't care enough about it to bring their grade up for it.

Overall it wouldn't be a good idea because people who want to be on a team that they are good at wouldn't be able to do what they want because of their grade.