I think that competency-based education should be used in high schools alongside traditional education, I think both approaches tackle different areas in education and they would work well together if they were used correctly, I think that more research should be done of the effectiveness of competency-based education compared to tradional education.
Competency based education works best for individual lessons, not for classes. It's good for developing concrete skills, and some people learn those at a faster rate than others. But that is why there is homework and practice outside of class. A decent high school curriculum has to also teach abstract subjects, which don't lend themselves as well to independent study.
There needs to be certain standards that are set in high school that every one should have to perform at or otherwise not graduate. You can't just let people slack off in school, because they will probably end up on welfare. If you can't meet a minimum standard then you do not deserve a diploma anyways.
College is the place for competency based education. This could also be used in high school replacement programs like vocational technical schools. However, high school still mainly serves as a launch point for college programs. High school is still designed to instill fundamentals of learning and education required for most jobs. It isn't until college when a specialization is typically established and market norms for specific competencies can be identified.