set singleton instance to nil in my case

I am using OCMock 3 to unit test my iOS project.

I use dispatch_once() created a singleton class MyManager :

@implementation MyManager

+ (id)sharedInstance {
    static MyManager *sharedMyManager = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedMyManager = [[self alloc] init];
    });
    return sharedMyManager;
}

I have a method in School class which uses the above singleton:

@implementation School
...
- (void) createLecture {
  MyManager *mgr = [MyManager sharedInstance];
  [mgr checkLectures];
  ...
}
@end

Now, I want to unit test this method, I use a partial mock of MyManager:

- (void) testCreateLecture {
  // create a partially mocked instance of MyManager
  id partialMockMgr = [OCMockObject partialMockForObject:[MyManager sharedInstance]];

  // run method to test
  [schoolToTest createLecture];
  ...
}

- (void)tearDown {
  // I want to set the singleton instance to nil, how to?
  [super tearDown];
}

In tearDown phase, I want to set the singleton instance to nil so that the following test case could start from clean state.

I know on internet, some people suggest to move the static MyManager *sharedMyManager outside the +(id)sharedInstance method. But I would like to ask, is there any way to set the instance to nil without moving it outside +(id)sharedInstance method?