Our DS has not done traditional school sports, but has done Taekwondo for 10 years and is a 3rd degree black belt. Making a commitment and keeping it over time is important and I believe shows dedication. If the school schedule doesn't work for you, commit to TKD and excel at it. If you aren't on the sparring team, try it. Sparring is great conditioning and competing is a valuable experience.
The only thing that caught my eye was you stating you didn't do tennis at school because it took too much time. If it interferes with class schedules (weird that your school does that) I understand. If it is a matter of juggling 3 hour practices and homework, chores, commute etc., then you might have to get better at time management.
Juggling a hefty class load, sports and EC's well is part of what makes a candidate a great candidate.
Good luck to you, and congrats on your soon to be citizenship.