Help me understand...

[hauntedvagina]

... how primary schools will be managing close contact tracing after 19th July.

It's being managed by Test and Trace so how will they decide who needs to isolate? When does close contact isolation end for under 18's?

Schools won't be managing it as you say, t&t will. Although there is DoE guidance that schools can manage the process for the first two weeks after the end of their term, apparently. Parents will have to give what details they can about friends etc if asked. Contact isolation ends on 16th August.