Most kid-sized backpacks just aren't big enough for a normal laptop.
A messenger bag is a rectangular bag with a fastening flap on the front, and a shoulder strap. I have a couple of them.
Strangely enough, I do keep my laptop in a backpack. However... It was a freebie my Mom got from work, which is a technology company, and it was specifically designed for a laptop. Only issue is... I have a widescreen... So it barely fits! LOL... But it's great. My messenger bags don't have any padding. And as much of a klutz as I am...
Wish they would provide laptops for our kids. But then, they'd have to have 2,500 of them on hand for just the high school...