Added import statement Before. Moved school object instantiation in a setUp method.

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

+import org.junit.Before;
 import org.junit.Ignore;
 import org.junit.Test;
 
 import static org.junit.Assert.assertEquals;
 
 public class SchoolTest {
-  private final School school = new School();
+  private School school;
+
+  @Before
+  public void setUp() {
+    school = new School();
+  }
 
   @Test
   public void startsWithNoStudents() {
       // Such as UnsupportedOperationException when an unmodifiableMap is used
     }
 
-    assertThat(school.studentsByGradeAlphabetical().get(grade), 
+    assertThat(school.studentsByGradeAlphabetical().get(grade),
       not(hasItem(studentWhichShouldNotBeAdded)));
   }
 }