My son is 11, but a small 11, he was coming from a Specialized Riprock 20 (the previous design with suspension fork and tighter geometry), and so we initially went into LBS to look at 24" bikes. He is only about 54" tall, so we weren't sure he was ready for a full size bike. Happened they had a leftover Trek Marlin 5 in an XS, so 27.5" wheels, pretty dropped top tube. It's a little long for him, but that is perfect to give him room to grow into for a couple of years. Has mounts for a rack, and a nice integrated kick stand mount out back. So that will be nice for riding to school next year.
New or used I'd definitely try for a smaller adult bike than a bigger kids bike, I think it gives them more room to grow into. I remember my parents did something similar many decades ago, I graduated from a BMX bike to a Mongoose Supergoose, and rode that for a lot of years until late in high school when I got a Cannondale R400 road bike.