Security and privacy

Hello everyone,

i got some trouble with my setup of the user permissions of my moodle. What i want is not that difficult, but i didn't get it.
Imaging a school with different classes ( first, second, third and fourth). Now i have a four course categories for each class with several courses. I also set up four global roles for each. Every student is part of the role corresponding to his class. The students should only see the courses of his class.
How can i get moodle working like this?

regards,
Jan