My daughters (primary school) go to Wang (EC location) for their compo writing and it has helped them a lot over the years. We also have a tutor that comes to the house to supplement. Wang is not really the most personable, and you feel like your child is just a number, but for the compo writing, they stand out compared to the others we tried.
Friends that stay in Bishan swear by Berries. Not sure where you are, but you might want to check them out too. Good luck!