かがみの孤城 might be a good fit. It uses relatively simple, everyday language most of the time. The main characters are all middle school age (plus occasional parents, teachers, etc. show up). There are fantasy elements, but they are mostly there to create the desired setting. The fantasy aspects are often ignored (in favor of focusing on the characters and their interactions with one another) to the point that sometimes you might forget there was even a fantasy premise to begin with (I considered that a good aspect of the book by the way).
We actually just finished reading this as part of a book club, and overall people had a positive opinion of the book. Though keep in mind that it is on the longer side (like 550 pages), which could make it a bit tough depending on your existing reading experience. While the book club conversation drops off later on, discussion early on could help you if you’re having any trouble. And of course, you can always post and ask questions since many of us are still around and happy to help.