My initial reactions:

1. Yes, your wrestiling would almost certainly make putting on muscle difficult.

2. You are a junior in high school? Aim a little higher than just being a personal trainer. Any shmo can pay to be certified. If you REALLY want to have a long lived career in fitness and athletics, go get a degree that deals with this.

3. 5'8" 153lbs and 12% BF doesn't sound out of shape for me. Just as a reality check, being in shape isn't about 6-pack abs or any of that crap.

4. Putting on 10lbs of muscle at your age shouldn't be a major feat. You could probably wrestle and still manage to put on muscle post season.

5. Being muscular may help attract some clients, but unless you know your crap, they won't stay. See point #2. Get an education that is worth more than the $300 fee for certification.

6. Whether you wrestle or not should come from whether you enjoy it or not. You have a long time to sort out the rest of your life. Enjoy where you are currently at because you only get one shot at it.