One field for each one category - SOLVED -

Please,

try my last solution

Regards

Speaking of your last suggestion:
At the time of setting the permissions of a user in "flexiaccess" will appear the user and the group that user belongs.
Did I set the permission should always both: user and group?

No,
just set the permissions for the group only.
The idea is that if you set the permission for the group then you only need to assign the user to the groups

A use maybe assigned to more than one group if needed
Regards

I created two groups, teachers and students. At the moment I do tests only with these two groups.
In "flexiaccess" I have given permission for these two groups, in categories and menus.
The panel of permissions to each field "select" I give permissions to both groups.
When a student comes to see his notes, the student sees the notes of other students.
I must be doing something wrong.

no, you did nothing wrong, but something more is needed

How you distinguish notes between students? you have one category per student?

I suppose a note is one Flexicontent item, right?

Each student is a sub-sub-category of "flexicontent."
Main categories: the student's course (one course = one year of study)
Sub-category: type of data (note, behavior, interest ...)
Sub-sub-category: student's name.
To identify a student's note I wrote the student's name on the label of the field "select".
The note of each student is one of the fields that appear in each item of "flexicontent"