Children's Magical Book Series 2000s

I remember reading this book as a kid. I think it might be from 2005 or 2006. A boy and girl are tutored in magic, I think another boy is also tutored with them but he is an elf. I remember the boy travels by rainbow to the magical world. There are homes in trees, with the wealthy in very old trees I think. Their tutor is also a main character. That's all I remember :(