Perfect alignment between your personal/professional purpose (mission) and the purpose of the organization (school, district, etc.) you work for is unlikely.
Where purposes collide, I think you have three options.
Option 1: Adapt
You can work to adapt your purpose to fit more comfortably into the school's purpose. This may require some difficult decisions on your part.
Option 2: Adopt
You can adopt the school's purpose into your personal purpose. This option allows you to direct how to incorporate the organizational mission into your professional mission.
Option 3: Abandon
The third option is to abandon one of the two purposes: yours or the school's. Abandoning the school's, in reality, means choosing to find another school to serve that is a better fit for your work. Abandoning your purpose means replacing your purpose with that of the school. I am not sure one can completely do this as it requires giving up any personal mission. Even if you try, I suspect you will be perpetually frustrated and your work will suffer. In the end, leaving the school (either by your choice or the school's) is likely to happen.