If the charter school offers an effective, individually focused education then why is it testing low? How low?

Tests scores reported in greatschools are usually relative to just-adequate state standards. If the school's approach can't even get the kids to that level, then I would wonder what was good about it.

Now, if the charter school happens to draw kids with learning challenges, or who have been underserved in the past, then maybe it is an excellent school despite lower scores. But I wouldn't automatically conclude that a school that does a great job bringing low performing student closer to grade level is necessarily a good fit for a child who is already excelling.