How do you say my school in the plural form?

You can't say "my school" in the plural form because it's already in the singular form. "My school" refers to one specific school.

If you want to talk about multiple schools that belong to you, you could say:

* My schools (if you attend more than one school)

* The schools I go to (if you attend multiple schools)

* My different schools (if you want to emphasize that they are distinct)