Universities don't really care. They are gonna look for better achievements. Do you ever do international competitions for sports for example? Those are more impressive so they will look at those. School captains don't really do anything so it won't impress the people who are selecting from a bunch of candidates.
What they would morseo look for is the depth of your involvement and what you have done. So yes they would consider your duties as a School Captain for scholarships and what not, but they would want some substance from it (so what you did and how much involvement you had as a good captain).
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Also on the topic, generally you will have to do mainly ushering responsibilities during school events, speak at assemblies and give advice. However there is much more that can be done outside of this, which is entirely within your hands. You can discuss certain school policies with staff, bring in new initiatives towards making the school community better in general, increase awareness of school events and policies for other students and parents, build a more integrated school community, introduce new and fun school events, host fundraising events (went really damn well for our formal budget raising about $2000 at some 5 manned bbq's in just one day and we did it a few times), etc.
There's a lot that you can do but it's just about how you approach the role. You can do bare minimum and get by fine with the role (not really much to it if you do bare minimum), or you can take advantage of the situation and do the best for the school as a whole.
Also, if you do take advantage of the situation, you might be able to find yourself becoming a much more proficient team worker and leader. Again, it all depends on how you approach the role.