Answer:
Because learning more about the world helps you make good decisions for yourself, which helps you to be happier in the long run. Better educated people are not always happier, but the more you know about the world the more options you have and the harder you are to take for a ride.
It also helps you realise that “Why school is important” is a statement rather than a question - it’s not important in the grand scheme of things - after all, the meaning of your question is clear enough, but people do tend to judge on these things. It’s unfair but its true.