I disagree with most other reviewers. This didn't feel like Harry Potter at all to me, though maybe the publishers hoped it would. (In general, it's weird that so many readers act as if HP was the first book about a child discovering that they have magical powers or are inducted into a magical world, but I digress.) If anything, this felt more like a Buffy the Vampire Slayer-esque story set in Nigeria: as much as they received some instructions from their teachers and mentors, they were sent by them to battle with the Big Bads without knowing everything they needed to.
Much has been made about the world-building, but the characters were fantastic as well, particularly Sunny and her band of "Scoobies": serious Orlu, brilliant and insouciant Chichi, and defiant Sasha. Everything about the characters and their interactions made sense, which made their actions seem character-driven.
What I loved most about the world-building was that what we consider abnormalities are in this world signs of greatness, whether it's an albino girl being able to make herself invisible as well as travel between spirit and physical worlds, or someone with ADHD being able to "undo" powerful magic. That "perfection" is over-rated is an important message for any age group.
My only disappointment was that the final battle with the ultimate Big Bad felt a little flat. I would have liked to have seen more of a buildup. Here's hoping that's more drawn out in the next book.