Leagally no, not if working in close vicinity of the teachers and other CRB checked staff. If working on their own with children, without other adults around I think they are supposed to be.
I am helping a couple of days a week in DD's school as a volunteer teaching assistant. I am currently being CRB checked so I can get more involved.
I did similar, one day a week, from easter to the Summer and didn't do the CRB bit then, but was never alone with children either.