Primary school teacher here.
Agree with making sure child knows what is in their bag. The number of times you ask a child if they have X Y or Z in their bag and they look at you blankly and say they don't know, so we go to check bag and voila! it was there all the time!
Also agree with name labels in EVERYTHING, including shoes, plimsolls, coats, water bottles, glasses case if they wear glasses, snack box if they bring one, sunhat/baseball cap, etc. When I've handed out water bottles at hometime to an EYFS or KS1 class, normally on average about 4 bottles out of 30 have actual names on, so you hand those out first and then you're just endlessly repeating, "Whose is the dinosaur one? The unicorn one? The Spiderman one? The pink one? The other pink one?"!!!" There were SO MANY unclaimed bottles at the end of last term. Massive Brownie points for all parents who actually name them.
And don't get me started on parents who say, "He's lost his jumper, it's red" (all the 100 unnamed jumpers in lost property are red).
Always have spare pants in the bag, even the best toilet trained children can have accidents in Reception, usually because they're so busy playing they forget to go to the loo. And a plastic bag for any wet clothes.
And teachers will love you if your child regularly returns their reading book and the reading books don't disappear home and never return, or don't return for weeks, or come back looking like they've been through the washing machine...
Also check all your child's pockets when they get home to weed out random toys they have pocketed from school or disgusting objects they picked up in the playground.