When you change the site setting, you can choose whether you want to enforce it for existing users.
So you should be able to remove support and school_a from the setting, save without applying that for existing users. Then you wait a few minutes and add them again. This time, apply the change to existing users. (Don’t be scared because of the count, it might be wrong because you are adding more than one category)
There is a guide here, but it is focused on overriding any customizations, enforcing the same sidebar for everyone: How to set the same sidebar categories for all the users.