Does the student incur sin if the teacher raises his grade due to his good manners or for no reason, without a request from him, and is he obliged to object to that? And do the students incur sin if the Ministry of Education or the school administration raises their averages without their deserving it, despite their unwillingness?

The ruling on increasing students' grades hinges on whether it conforms to the established standards and regulations. If it conforms, then there is no impediment to it, and the student has no right to reject it. However, if it violates them, then it is impermissible, and the student must denounce it and refuse to accept it to the best of his ability.