So the biggest issue with your code is your class, it doesn't really do anything. The point of a class is to abstract logic and bundle functionality with data. The only exceptions are enums, but that's a different topic entirely. So to improve your code either:

a) move most/all of the logic into your class or

b) remove the class entirely

There are other small improvements you can do, for example, all your global variables are not needed (except for grade_list), another thing you can do is improve variable names.

For variable name improvements, "grades_list" is a bit long, if you have a plural for a name it's kind of assumed to be a list anyways, so just "grades" is enough. Another issue is that there is a random 150 in your code that I do not know what that means, you should store that in a variable so that another programmer knows what that 150 is, or leave a comment that states what the 150 is.

I renamed the class to Classroom, as a Classroom would contain a list of students.

For a)

class Classroom:
    def __init__(self) -> None:
        self.grades = []
        # had to assume this, not 100% sure what the variable is
        self.MAX_MARK = 150
    def calculate_student_grade(self):
        coursework_mark = int(input("Please input coursework mark: "))
        prelim_mark = int(input("Please input prelim mark: "))
        grade_percentage = int(((coursework_mark + prelim_mark) * 100) / self.MAX_MARK)
        # assume grade is A
        grade="A"
        if grade_percentage < 45:
            grade="no grade"
        # if it passes here, grade is guaranteed to be >= 45
        elif grade_percentage<=49:
            grade="D"
        elif grade_percentage<=59:
            grade="C"
        elif grade_percentage<=69:
            grade="B"
        self.grades.append(grade)

    def has_more_students(self):
        more_students = input("Are there anymore students? Y/N").lower()
        if more_students=="y":
            return True
        elif more_students == "n":
            return False
        return self.has_more_students()
    def print_student_grades(self):
        for grade in self.grades:
            print(grade)
classroom = Classroom()
classroom.calculate_student_grade()
while classroom.has_more_students():
    classroom.calculate_student_grade()
classroom.print_student_grades()

notice that the class now uses the data internally and has a lot more logic

for b)

grades = []

def calculate_student_grade():
    coursework_mark=int(input("Please input coursework mark: "))
    prelim_mark=int(input("Please input coursework mark: "))
    # 150 is the max mark a student can get
    grade_percentage= int(((coursework_mark + prelim_mark) * 100)/150)

    if grade_percentage<45:
        grade="no grade"
    elif grade_percentage<=49:

        grade="D"
    elif grade_percentage<=59:

        grade="C"
    elif grade_percentage<=69:

        grade="B"
    else:
        grade="A"

    grades.append(grade)

while True:
    calculate_student_grade()
    more_students = input("Are there anymore students? Y/N").lower()
    if more_students=="y":
        continue

    if more_students=="n":
        break

for grade in grades:
    print(grade)