Detecting a particular user input without first processing it in a while loop

I made few changes -

1. Initialization of minValue and maxValue , in your original code, minValue was -1 and grade could not be less than -1, hence you always saw minValue as -1. In general, if you want min and max vales, it is better to initialize like I did here.

2. Moved the logic of calculating grade, average AFTER user has entered the grade in order not to mess up the average.

3. Your original code was:

if (grade < maxValue)

I changed this to be if (grade < minValue). I believe it was a typo.

import java.util.Scanner;

public class Lab4_EnteringGrades {
    public static void main (String a[])
    {
        Scanner kb = new Scanner(System.in);

        double average = 0;
        double gradesSum = 0;
        int grade = 0;
        int numTests = 0;
        int maxValue = Integer.MIN_VALUE;
        int minValue = Integer.MAX_VALUE;


        while (true)
        {

            System.out.print("Enter a grade: ");
            grade = kb.nextInt();

            while ((grade < -1) || (grade > 100))
            {
                System.out.println("Invalid grade.");
                System.out.print("Enter a grade: ");
                grade = kb.nextInt();
            }
            if (grade == -1) {
                break;
            }

            gradesSum += grade;
            numTests += 1;
            average = gradesSum / numTests;


            if (grade > maxValue)
                maxValue = grade;

            if (grade < minValue)
                minValue = grade;
        }

        System.out.printf("\nTotal number of tests: %d", numTests);
        System.out.printf("\nThe average is %.1f", average);
        System.out.printf("\nThe max value is %d", maxValue);
        System.out.printf("\nThe min value is %d", minValue);
    } }

The problems are that

1. You do gradesSum += grade; before reading the first grade, when grade is still 0.
2. You do if (grade < maxValue) after the user has entered grade=-1;, thus always find that -1 is smaller.
3. You compare the grade to maxValue both for checking max and min
4. Since you initialize minValue=0, you won't ever find a smaller value (except -1), so you should initialize it to something larger

You can move all the code that processes the grade until after you've read it, and then break if the user enters -1:

import java.util.Scanner;

public class Lab4_EnteringGrades
{
public static void main (String a[])
  {
  Scanner kb = new Scanner(System.in);

  double average = 0;
  double gradesSum = 0;
  int grade = 0;
  int numTests = 0;
  int maxValue = 0;
  int minValue = 101;

  while (true)
     {
     System.out.print("Enter a grade: ");
     grade = kb.nextInt();

     while ((grade < -1) || (grade > 100))
        {
        System.out.println("Invalid grade.");
        System.out.print("Enter a grade: ");
        grade = kb.nextInt();
        }

     // Stop if the user entered -1
     if (grade == -1) break;

     gradesSum += grade;
     numTests += 1;
     average = gradesSum / numTests;

     if (grade > maxValue)
        maxValue = grade;

     if (grade < minValue)
        minValue = grade;
     }

  System.out.printf("\nTotal number of tests: %d", numTests);
  System.out.printf("\nThe average is %.1f", average);
  System.out.printf("\nThe max value is %d", maxValue);
  System.out.printf("\nThe min value is %d", minValue);
  }
}

The while(true) { if(..) break; } is slightly awkward though. You can improve the code further by creating a separate function for reading input:

import java.util.Scanner;

public class Lab4_EnteringGrades
{
  public static int readGrade(Scanner kb) {
    System.out.print("Enter a grade: ");
    int grade = kb.nextInt();

    while ((grade < -1) || (grade > 100))
    {
      System.out.println("Invalid grade.");
      System.out.print("Enter a grade: ");
      grade = kb.nextInt();
    }
    return grade;
  }

  public static void main (String a[])
  {
    Scanner kb = new Scanner(System.in);

    double average;
    double gradesSum = 0;
    int grade;
    int numTests = 0;
    int maxValue = Integer.MIN_VALUE;
    int minValue = Integer.MAX_VALUE;

    // Common idiom for reading a grade until it's -1
    while ((grade = readGrade(kb)) != -1) {
      gradesSum += grade;
      numTests += 1;

      if (grade > maxValue)
        maxValue = grade;

      if (grade < minValue)
        minValue = grade;
    }
    // Consider handling the case of numTests == 0
    average = gradesSum / numTests;

    System.out.printf("\nTotal number of tests: %d", numTests);
    System.out.printf("\nThe average is %.1f", average);
    System.out.printf("\nThe max value is %d", maxValue);
    System.out.printf("\nThe min value is %d", minValue);
  }
}