I'm calling it a music bag but it would work just as well as a school book bag. it measures about 14" wide and 11" tall and the clever bit is the closure. The handle threads through a gap in the flap to hold it shut. It took me quite a lot of measuring and thinking (and measuring again!) to work out how to do that.
The pattern for the Music Bag is free to download but, if you enjoy making it, do click on the fundraising tap on the sidebar to donate to my chosen charity.