I work in reception. Water bottle a must, we encourage the children to drink throughout the day, especially if hot, the school can't afford to provide disposable cups apart from the odd occasion when a child forgets their bottle and we don't have the time to be washing cups everyday (which get knocked over anyway).
No rucksack needed, just bookbag, it goes in a drawer and we put all newsletters, work etc. straight in them. Rucksacks just take up space we don't have.
Sun hat for sunny days, raincoat for wet, we go out in all weathers and it always amazes me how many children don't have these.
And yes, label everything, the amount of times I have parents complaining about lost jumpers, 'oh, I think the name washed off/tag fell out/hadn't got round to sewing it in'. We have 60 4 yr olds all taking off their jumpers, of course it will go home with the wrong child if not named.
Also, I took 20 pairs of pants in to use as spares last year, by christmas we had none left, so please do return any your child comes home in.
I also have 3 dcs of my own, M&S are great for shoes, lasted twice as long as far more expensive shoes on my dc and they say they're really comfy.