Can a user be both Student and Grader?
Student and Grader are both both Primary roles
A user cannot have more than one Primary role. See Roles miniguide,
It occasionally happens that the grader is registered as a student through Minhal Talmidim.
A student registered through Minhal Talmidim cannot be unenrolled from Moodle.
The solution is to assign the student one of the Utility user roles. See What is the Utility user role?
Then to grant the appropriate capabilities to that role.
For example, capabilities mod/assign:grade and mod/assign:viewgrades in order to grade Assignments, See Capabilities miniguide