The school should issue you with a list in the summer, so hold off buying anything until you have that. Also, they are growing quickly and if you buy it too soon, some things might not fit in September. I'd say go to the school shop around the end of June/early July - early August and they are run out of everything here. If it doesn't have to be bought from the school shop, buy it elsewhere as it will be much cheaper. DD didn't like the PE skirts in the school shop as she thought they'd be sweaty, but luckily I knew I could buy a cotton one online from M&S £6 cheaper!
I think most of it is covered above, but DD's school like them to have a foldable 30cm ruler which fits in their pencil case - I got DD's from Tescos for about £1. When you go to parents evening ask them what calculators the school use (my DDs also use scientific Casio ones) - if the teacher is showing them a symbol to use at the front of the class it will be handy to have the same calculator so they can see which one they need to use easier. A school apron may be needed for cooking.
DD went through two fashion bags in the first term, by that time she was happy to have a backpack as she realised a lot of friends had swapped over to them. Got hers from Animal and it's lasted two years - has a bit of life in it still actually, but got DD another one at Xmas.
My DD was happy to take sandwiches as all the girls from her school were taking them. She goes to a large school, the boys seem to like school dinners and buying sandwiches at school. The girls aren't willing to spend 30 mins queuing for food, as they'd rather be chatting, therefore, they take sandwiches. If your DC knows others going to the same school, get an idea what they'll be doing the first week or so.