input: Write a python function to calculate the average age of a list of Student objects.
  assert Student.get_avg_age([Student("Alice", 20, "F"), Student("Bob", 22, "M"), Student("Charlie", 24, "M")]) == 22
  assert Student.get_avg_age([Student("David", 18, "M"), Student("Eve", 19, "F")]) == 18.5
  assert Student.get_avg_age([Student("Frank", 21, "M"), Student("Grace", 23, "F"), Student("Heidi", 25, "F")]) == 23
output: class Student:
      def __init__(self, name, age, gender):
          self.name = name
          self.age = age
          self.gender = gender
      
      @classmethod
      def get_avg_age(cls, students):
          total_age = sum(student.age for student in students)
          return total_age / len(students)