because they are still growing, with bones, muscles and ligaments that can be easily damaged, plus all the heavy books and trudging up and down stairs, etc, I think the rucksack is one of the most important items of school kit.
I know that good rucksacks are expensive and it's Christmas and you may not be able to afford one. But you could wait for the sales?
I got DS (Yr7) a NorthFace Recon. I did a load of research about which were the best to support the back and would be comfortable to carry.
My priorities were: ergonomic as possible; padded shoulder straps; padded back; straps to do up around waist and chest. Plus the obvious ones of having a laptap compartment (not for laptop but for putting A4 size books into), it being waterproof.
I am also aware that the bigger they are the more chance there is that they will just bung everything into it, so I didn't want to get an enormous one either. FWIW, try going into a huge outdoor shop like Cotswalds or Northface or Blacks or Millets and look at what they have. When you and DC find one you like, look on line for reputable cheaper sellers.
Or you could try TKMaxx, but only go for good quality brands because cheaper ones usually are less reliable.
Good luck.