why don't schools add Meditation - Mindfulness to its curriculum?

2 Reasons:

1. They are stupid to do anything that wise and beneficial to the world.
2. You can't force it or grade it. Meditation comes from within. When 3rd parties rate it it cease to become real meditation. It also requires a willing mind which already understand the importance and benefits of medicine.

Teaching meditation is like teaching creativity. You can only show the way.