grade-school: add hints to @Ignore annotations

exercises/grade-school/src/test/java/SchoolTest.java

     assertThat(school.numberOfStudents(), is(0));
   }
 
-  @Ignore
+  @Ignore("Remove to run test")
   @Test
   public void addsStudents() {
     school.add("Aimee", 2);
     assertThat(school.grade(2), hasItem("Aimee"));
   }
 
-  @Ignore
+  @Ignore("Remove to run test")
   @Test
   public void addsMoreStudentsInSameGrade() {
     final int grade = 2;
     assertThat(school.grade(grade), allOf(hasItem("James"), hasItem("Blair"), hasItem("Paul")));
   }
 
-  @Ignore
+  @Ignore("Remove to run test")
   @Test
   public void addsStudentsInMultipleGrades() {
     school.add("Chelsea", 3);
     assertThat(school.grade(7), hasItem("Logan"));
   }
 
-  @Ignore
+  @Ignore("Remove to run test")
   @Test
   public void getsStudentsInEmptyGrade() {
     assertTrue(school.grade(1).isEmpty());
   }
 
-  @Ignore
+  @Ignore("Remove to run test")
   @Test
   public void sortsSchool() {
     school.add("Kyle", 4);
     }
   }
 
-  @Ignore
+  @Ignore("Remove to run test")
   @Test
   public void modifyingFetchedGradeShouldNotModifyInternalDatabase() {
     String shouldNotBeAdded = "Should not be added to school";
     assertThat(school.grade(grade), not(hasItem(shouldNotBeAdded)));
   }
 
-  @Ignore
+  @Ignore("Remove to run test")
   @Test
   public void modifyingSortedStudentsShouldNotModifyInternalDatabase() {
     int grade = 2;