I'm not sure who Aku specifically is. Aku could be a person with a great imagination who tells all kinds of stories, like adventure stories, fairy tales or even true - life stories.
One idea could be 'Sunrise Academy'. It gives a sense of new beginnings and hope, which could be great for a novel set in a school environment. The students could be seen as the 'rising sun' of the future.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
The character of Aku could be developed in a 'pregnant Aku in Samurai Jack fanfiction' by exploring how he interacts with the world around him differently. He may become more reclusive as he doesn't want others to see his 'weak' state. Or he could become more aggressive in trying to protect his pregnancy, which would be a new aspect of his character. Also, his relationship with his own power might change. Maybe he has to find new ways to use his magic that don't harm the unborn, which would force him to grow and adapt in ways he hasn't before.
Sherborne School. It has an old - world charm with its ancient buildings and rich traditions. This can add a touch of mystery and history to a novel set there.
A good name could be 'Mystic Grove School'. 'Mystic' gives an air of mystery and 'Grove' makes it seem like it's in a peaceful, nature - surrounded area. This would be perfect for a novel where there are secrets to be uncovered in the school grounds.
One idea could be a crossover where Aku accidentally opens a portal to the Naruto universe. Naruto, being the curious and brave ninja he is, decides to explore Aku's world. They could start off as enemies, but gradually come to understand each other's motives and end up teaming up against a greater evil that threatens both of their worlds.