Keep some change handy at home for non uniform days, bake sales etc. Change/correct money is always welcome.
Some schools have old newsletters online, so check your allocate school as this may have give you an idea as to the type of events the school normally has.
Many schools will do the same type of events eg Christmas jumper day and World Book Day, so you know these events are coming well in advance.
Keep a small stash of craft items like kitchen roll tube, yoghurt pots, glue, paper etc, as the children often need to make something.
A mans t shirt can often be repurposed for many dress up days. My DN needed to go i dressed as a Celt, so I used one of my OHs brown t shirts as a tunic, with a belt. In fact, a brown t shirt will work for most historical eras.
If you can't bake, shop bought cup cakes or doughnuts etc are a much welcomed donation.
Any unwanted Xmas gifts eg toiletries can be donated for the school fayre, so keep them aside.
Some schools sell second hand uniform, so put aside any good condition uniform you no longer need.