Both ACE and ACSM are highly recognized by employers. You may want to consider it sometime in the future, but I would still start with ACE, ACSM, or possibly Aerobic Pipeline. The CSCS is becoming much more popular in the personal training circle and it certainly seems suitable if you want to be a strength coach, but it may not be in line with your training philosophy. Many offer 1 day certification programs for their staff that you may be able to participate in. Though it probably will not be the ACE exam I just mentioned, you may find a few smaller-scale programs that would be helpful.
Colleges sometimes offer longer, more in-depth fitness certification programs as well.
I just want to make one more comment about your question. I am seeing more and more of a need for qualified and competent strength coaches at the high school level. Working with teens is very exciting - and they need good trainers desperately. Maybe you can help set a safe and effective training precedent for other high school athletic programs. Thank you for taking the time to review my question with earnest.