public String getStudentGrade(String aNumber)
{ // begin getStudentGrade method
//You will need to loop through the ArrayList dataStore to retrieve the specified Student object
    for(int i=0;i<data.size();i++) //loop through all of the students in the list
    {
        Student aStudent = (Student) data.get(i); //get the student in the i'th position
        if (aStudent.getPNumber().equals(aNumber)) //is it the student being searched for?
 	    {
  	        return aStudent.getGrade(); //if so then return it's grade
        }
    }
	return -1;  // This will be done if the cat is not found to inform the program
}