Decertify the union as it was involved.
Ban every teacher, administrator and principal identified from working in schools for life.
Make it impossible for this type of cheating to occur again.
This demonstrates the validity of arguments against teacher bonuses. They reward teachers who cheat. Another issue is that who's enrolled in a class for the tests can (and is) used to make some teachers look good and others to look bad.
The best way to improve performance is to have excellent even handed discipline and to have as few transfers as possible for teachers, administrators and students. Stability, high behavioral expectations, and broad curriculum requirements being met would work just fine.