I run a site for students and teachers in a school.
All of them can write in all categories they have access to through Joomla ACL.

This is what I'l like to do :
allow a few people to edit all elements, not in all categories they can write, but in one category only.

Is there a way to do this ?

Thanks!

Unfortunately no,
Each category the user has access to is both for editing and adding content.