Consider the school's total number of students and its average class size. The smaller the class size, the more personalized attention each student can get from the teacher and the fewer total students, the more attention each student can get from the administration. Also consider the available extracurricular activities to see if students have the ability to explore new things.
Additionally, look at the school's college acceptance rate, and if available, look at which colleges the students are being admitted to. Look at what courses the school offers, including AP and IB classes that can earn students college credit. If there are AP courses, check to see how many students take these courses, and of those students, how many take the AP exams and what their average scores are. Some schools also offer their students' average SAT and ACT scores, which can tell you about the school's academics and environment and whether the school's students are those who take their academic careers seriously.
The reputation of the school's sports teams may also be important to take note of if sports are important to you or if you hope to get a sports scholarship to college. Finally, if it is relevant, look to see what kind of special needs programs are in place at the school and how easy or difficult they are to access.