The O Level thing isn't a "special treatment for MP's son" scandal. It was a private arrangement with the school, where the school would agree to do what they could to let him take the test, and Mr Merrick would agree not to cause a scandal with the school. In Run Away Home, Patrick says that someone who thought on official lines said his O Level probably wouldn't count. (There are precedents for taking O Levels out of the proper time, most commonly when you have a clash of papers, but they generally depend on having been under direct supervision of a teacher throughout.)