What is the interior material like? Is it thin and gauzy or more durable? I'm worried about holes developing. Thanks!

I've carried mine for quite some time and the lining seems to be durable, I've not ripped or worn any holes. It is cloth but not gauzy and flimsy. I would purchase a SNUG STAR again, even this one if I felt the need.

Is this machine washable?

I tossed mine into the washing machine & it did very well. Helped to soften the canvass. I did let it air dry. Did not want the zippers banging on my dryer

Would this be a good school bag?

Depends on what you have to carry. It's sturdy but I don't think it would hold the pile of books we carried when I went to school. ;) If you are digital and just need your tablet and lunch then yes.

How long is this bag from top zipper to bottom? the measurements given are unclear.

The bag is about 14 inches tall, but the zipper compartment is inside the bag and is only about 5 inches up from the bottom. That is one small drawback to the bag, but it is still a fun and roomy bag. It has a couple of other small zipper compartments, that will hold keys, ID badges, or maybe a cell phone.