I got DD A4+ sized coloured pouches, for her main subjects - to put the books, copies, loose sheets etc all together. I coordinated her timetable (copies for locker, school journal, and home) using the same colours, and managed to find some of the same colours for hardback copies (required for some subjects) and regular copies and folders for loose papers and study notes. Just to make it easier to be organized when moving from classroom to classroom, and taking so much home at night and back in again after study.
Otherwise, she still had pencil cases that were fine, and a maths set (but got a new one anyway as DH organized the booklist - I'm keeping one at home for study and the other will live in her locker).
New scientific calculator was needed (type was specified on booklist).
We have a stash of red, blue and black pens, pencils, erasers, parers etc - built up over the years of primary and also to have plenty in the study/office when any of us need them. And I tend to use other coloured pens at times (for marking up different versions of things and seeing that I'm looking at the right one!) so DD can grab some of those too.
We added highlighters to that this year, and post-it notes.
We got her a stapler and 2 hole punch for school, as she needs to have folders there. We already had some at home as well, and loads of poly-pockets to put loose sheets into.
I got her a normal refill pad (lined), and also a plain one (no lines at all) and a boxed one (for graphs etc). I'll make use of those over time, but I thought it was better to have them when she needed them urgently. They're at home for now - but in her desk.
Apparently she needs other pencils for art (some different levels of B ones - different softnesses of the lead to make different effects when drawing) but we don't know exactly what yet.
The journal was provided by school and apparently the maths and the science copy books are to be bought there too.
Her old water bottles are fine, and as they have hot dinners in house, no lunch box was needed but she does need a small snack so smaller pots for fruit are being commandeered from my cupboard!