Mocking behaviour resets after each test with PowerMock

I'm writing unit tests using PowerMock, mocking behaviour of some util classes. Defining behaviour once for test class (by @BeforeClass annotation) causes:

• first test invocation to return mocked value
• second test invocation to return real method return value

Sample code:

import org.junit.Assert;
import org.junit.BeforeClass;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

@RunWith(PowerMockRunner.class)
@PrepareForTest( {A.class, B.class})
public class TestMockedMethods {

private static B b;

@BeforeClass
public static void setUp() {
    PowerMockito.mockStatic(A.class);
    PowerMockito.when(A.getVal()).thenReturn("X");

    b = PowerMockito.mock(B.class);
    PowerMockito.when(b.getVal()).thenReturn("Y");
}

@Test
public void test1() { // PASS
    Assert.assertEquals("X", A.getVal());
    Assert.assertEquals("Y", b.getVal());
}

@Test
public void test2() { // FAIL
    Assert.assertEquals("X", A.getVal()); // actual="A"
    Assert.assertEquals("Y", b.getVal()); // actual="B"
}

}
class A {
  static String getVal() {
    return "A";
  }
}
class B {
  String getVal() {
    return "B";
  }
}

Any ideas why second test is failing?