20
I don't think this scheme of grades encoded in the user name is a good idea. Here are some toughts:
Does this mean that when they have a higher grade, they will have to have a new login name?
Or they get membership of the additional user groups and their user name becomes unrelated to their grade?
If they can autocomplete their registration, without admin approval, then they can choose their level by incrementing the grade part of their user name and accessing files from higher grades.