This is very common catastrophe among school bags.
It is good to know it is very easy and low cost problem to get fixed up at home.
Unpick the seam.
Unpick everything. As you can see originally the straps were not fastened proper. Very common mistake the factories do. They save time and trouble at the most important part of the back bag.
Tidy it all up.
Fasten the straps first.
Then fasten the band.
Secure the band and at the same time you create extra stitches for the straps at the point from where they usually get tear out.
Fasten then the lining / cover / lid structure.
Now you have there actually 3 stitches to hold the straps.
Bias tape forms out the 4th stitch. It makes actually 2 more than this school bag originally had. Usually they sew the straps by one stitch and then it all by this bias tape. I use it only to neaten the seam allowance to be hidden.
It took me actually 30 minutes to fix the school bag and take the pics for you. From the 1st second I started to the last one when I stopped. Very cost effective skill to learn!
