CTY is a nice school supplement but not a replacement for school instruction. If your student has the time in his/her/their schedule, it might be a nice way to stay fresh with Mandarin, assuming you don’t speak it at home. Otherwise, your student will save time and benefit the most from taking a language offered by the school at school.
High schools are generally very specific about what kind of alternative credits they will and won’t accept and what graduation requirements must be met, so check with your student’s high school before making plans outside of the school for assumed credit.