Speaking as an infant teacher, don't go for too much! I've seen some little Receptions with lunch boxes bursting at the seams! I look in them and think "I couldn't eat that much!"
Particularly in those first few weeks, as they adapt to the noise of the dinner hall and the new routine, keep the lunch reasonably small. The children are very good at reporting home if the lunch was too small, but aren't so good at reporting if it was too big, and some can find a mound of food quite overwhelming.
4 items is more than enough. 1 sandwich, perhaps into 4 squares, but even 2 squares can be enough, particularly at the start of the year.
Something like a yoghurt is good, although I'd advise against Frubes, as Littles can find them hard to open. A "normal" yog in a pot, with a disposable plastic teaspoon is a better bet. If the spoon comes home, great, it can be washed and re-used. If it doesn't make it home, no biggie.
Something fruity, some apple slices, a banana, a pealed Satsuma, grapes, or cherry toms. Something prepared for small hands/mouths.
And a "treat" of some sort. A small bag of crisps, cookies, or something like a kitkat or penguin.
Oh, and a drink. A juice carton is fine if there is plenty of other opportunities for the children to drink if they are thirsty during the day. But if there isn't, or you have a "thirsty child", a larger plastic bottle might be better.