I intend to drop my players on the harbour and give them 12 days to reach the Exam House before the Imperial Exams are over. This will be the start of the campaign hook, as either they work together to ford the rivers of humanoids, or inevitably they are late and miss the exams - from the harbour it takes exactly 11 1/2 days if they don't get lost to reach Exam House. Once they're at the exam house they're going to take a skill challenge.
The first is a DC25 history question, "Why was Loue Boss assassinated by his nephew in the era of restored tranquility?" Which just introduces the rules of the skill challenge. (One person is the primary skill user, other team members make DC 10 skill checks using other skills to reduce the main DC by 5). So for example, the cleric is the one answering the essay question, but the bard uses deception to make the essay sound more intellectual than it actually is, the barbarian makes an insight check to realise Loue Boss wasn't murdered, he let his nephew kill him e.t.c.
The second challenge is a mock police interrogation, where it becomes apparent that the police interrogator is looking for a confession regardless of whether the suspect committed the crime or not. If they get the suspect to confess; it's a success. If they confront the interrogator successfully, also a success. The meta-element of the Imperial Exam is that they're not just testing ability, but also personality.
For example, one of the tests is literally the invigilator telling the examinees to jump off a bridge into a dark pit (it's a very short drop with a safety mat and an illusion cast over it). They can use their skills to question the invigilator, they can use their skills to investigate the drop, they can refuse the test or just jump in regardless - and this kind of info'll be used to assign them to the nobility, military, government or the Internal Security Bureau (ISB). I've got a lot of tricks up my sleeve for this one, and I think I'll do a writeup once I actually manage to execute my dastardly schemes upon my players