Хорошо. Я нашел способ:
[Test]
public void PrintStudentReport_ClassRoomPassed_StudentListOfFive_WithAAA()
{
//Arrange
IClassRoom classRoom = GetClassRoom(); // Gets a class with 5 students
IStudentReporter reporter = MockRepository.GenerateMock<IStudentReporter>();
//Act
reporter.PrintStudentReport(classRoom.Students);
//Assert
reporter
.AssertWasCalled(r => r.PrintStudentReport(
Arg<List<IStudent>>
.List.Count(Is.Equal(5)))
);
}