IMO, it is hard, but worthwhile to examine the causes of "self selection" and make changes to be more inclusive. If a specialty club attracts members that do not represent the demographics of the school, the adults need (IMO) be vigilant to make sure that this is not the result of problems in the club's organization, goals, interpersonal interactions, or some other factor not related to the specialty interest. Example: Here in my area almost all chess tournaments for elementary students are on Saturday. There are some rated quads on Friday nights, but that is all. This makes chess less appealing to families that have religious, educational, or cultural obligations on Saturday. These families children may attend the Thanksgiving tournament or some chess camps, but they will not be able to become truly competitive in tournaments. Adults helping to run the school chess club may only see that they have a "self selecting" membership that is very interested in tournament play and is free on Saturdays, but they could choose to stop and examine the reasons behind the selection. What is the focus of chess club? Will more students join chess club if the focus of the club is tournaments or by having a broader focus? Are there hidden costs to joining chess club (travel, entry fees, etc). Do students lack transportation to club activities because both parents work and may work on weekends? Are the club meetings held at a time that students that take the school bus can attend?
All of these factors mentioned above help to determine the "self selecting" population of the club or activity in ways that are clear and easy to discern.
This type of analysis seems more difficult when we start talking about gender (IMO), because adults may not know why girls and boys perceive themselves as unwelcome or "not a good fit" for a club. It may be simple and hard to solve: it seems to the child that "everyone else's dad works at [local high tech firm] and they can help. My dad doesn't" or "I don't have money for private music lessons and everyone else in jazz band takes them so…" It may be complex and hard to solve: All of the boys in math club talk over each other when trying to be the first to answer a question; the girls just go away rather than struggle with making their view heard. It may be something that I, as an adult, don't acknowledge is a real and substantial issue when you are young: there is only one student bathroom at the chess camp and girls have to share with boys; or there are no other girls at all.
Not sure what the solutions are. As parents, others have given good suggestions above, but I think the institutions need more sensitivity too.
