Inkheart series by Cornelia Funke was a huge hit with my Middle School students. The heroine is a little girl and it’s an action packed adventure story with lessons and “believe in yourself” kinds of motifs that really hooked my 6th,7th,8th Graders. I read a little to them each morning at the start of class and they were all (boys and girls alike) genuinely into it.
I no longer am teaching, but have recently read Arabat by Clive Barker which would also be a good middle/young adult reader series (two books). But, be sure to pic up the ones with the illustrations, they are integral to the story.
There are also the Septimus Heap novels by Angie Sage, a very nice middle reader series about a family of Wizards. classic good and evil stuff in an accessible middle school format.
For High School there is the Twilight Series by Stephenie Meyer, not a lot of “substance” but the books are engaging and move a several thread story line forward at a great pace.
I would definitely agree with the suggestion of the His Dark Materials by Philip Pullman novels for a “reread” in High School, I know many read it elementary, but there is so much they miss about those novels at the younger ages.
If you have advanced and bored high school students there is Milton’s Paradise Lost, Dante’s Inferno and Marion Zimmer Bradley’s Mists of Avalon. These would be for advanced and mature high school audiences as would require some in depth discussion.
For the crossover group 8th-9th grade try the Chronicles of Narnia by C.S. Lewis written for children, but the entire series is an engaging and time filling read. Without demanding too much, and perhaps just enough of the reader.
Good Luck!