As far as I know, joining the uniform team is complusory. It falls within the school timetable.
If the number of applicants exceed the max capacity of a team, there may be a lucky draw. Once you are allocated to a certain team, it is not easy to change. One of my boy's classmate has applied to change to grasshopper since last year but not yet succeeded.
I thought it is complusory as all my boy's classmates joined and the principal mentioned back in the 2nd interview that each kid should join one uniform team.