The error arises because you run subs.add(p1.first), which is a type str. p1.first does not have the method get_grade. What you want to run is: subs.add(p1) (the object which will have get_grade). Also, you can remove the redundant parentheses when defining the classes. You can write class Subject(): as class Subject:.

You can then change you add code to:

def add(self, student):
    if len(self.students) < self.number_students:
        self.students.append(student.first) # changed here
        return True
    return False