my dd finds the playground difficult too. she says there are too many people and they all run past her, then she can't move and gets bumped into. maybe it's the sheer mass of moving/running, noisy kids that she doesn't like. Does she have any close friends yet? Maybe you could talk about a game they could play together in the corner of the playground, like mummies and babies or fairies or something.
It must be pretty overwhelming for a small child. Does she have experience of lots of kids rushing around and shouting? Maybe reassure her that she isn't being naughty or odd for being scared (confronted with X primary age children I'd quake a bit as well) and try and find some ways of making things better? Does she have a friend she can play with for eg? And what are the staff doing about it?
dd1 has just started school and has had the same playground issues. Have you spoken to the teachers about it?
At dd's school they have a scheme where some of the year 6s buddy the younger children. After we spoke to the teacher dd was given a buddy to look after her in the playground and it's helped enormously.