You can practically write a manual on the answer to that question. There are several chapters to it, too: effective administrators and teachers, a clean environment, diverse curriculum and student body, excellent test scores and records, etc etc. All of it’s pretty black and white.
What’s not black and white is whether or not any school is good for your child. If you’re a parent, keep that in mind. Aside from the statistics, the football team, the teachers, the whatever, even the class environments, you have to consider the personality of your teen. This is mostly concerning the onset of high school, and even college. There are many diverse schools out there that emphasize certain things–such as the arts, humanities, science, music, even math. Lord knows you’d have a hard time finding a kid that actually likes math; but make no mistake, you’d find some kids like that. And if your kid is one of them, you don’t want to send that kid to a school with an excellent music program!
This helps your child fit in. If there’s something he or she enjoys or simply has a talent for, correspond that with the kind of school you’re dealing with. If your teen excels in writing, choose a school with a good English program. If it’s animals, you might want to find a school that offers many zoological immersion programs. The variety is almost infinite. Take advantage of it.
You see, it’s not just about the school’s credentials. It’s not just about the teachers. It’s not about how good the sports program is. It’s not even about the environment. What matters is what your child can do. Don’t just focus on good schools; focus on schools that are good for your teen.