Just to clarify, do you mean that they're given access to school facilities properly compensated in pay during the summer?

I have to ask since I was describing the extra summer work in the context of what they're paid to do, not what they do in their free time.

I expect most good teachers already spend their own time, especially during the summer, planning and preparing things for their students, but I'm arguing that it should be institutionalized and they should be properly recognized and compensated for their work.

If it's already like that then I think my local school district must be different. The high school parking lots are empty whenever I drive by during the summer.