为什么我的平均数求不出来，哪里出了问题

package assessment4;
import java.util.Scanner;

public class q4 {

public static void main(String[] args) {
    Scanner in = new Scanner(System.in);
    System.out.println("Enter the number of students :");
    int numberStudent = in.nextInt();
    while (numberStudent < 0) {
        System.out.println("Invalid student, try again :");
        Scanner sc=new Scanner(System.in);
        numberStudent = sc.nextInt();
    }
        int[] grade = new int[numberStudent];
        float sum = 0;
        for (int i = 0; i < numberStudent; i++) {
            System.out.println("Enter the grade for students " + (i + 1) + ": ");
            int num = in.nextInt();
            while (num < 0 || num > 100) {
                System.out.println("Invalid grade, try again ");
                num = in.nextInt();
                grade[i]= num;
                sum +=  grade[i];
            }
        }
    System.out.println("The average is :" + sum / numberStudent);
    }

}

你这个只能计算小于0，或者大于100的，如果输入的是0-100之间的数，里面的while是不会执行的；里面不执行，sum就固定是0