It says
"Public Health England data shows that outbreaks of suspected coronavirus in schools have tripled since they reopened to more pupils on June 1.
There were 44 respiratory outbreaks in schools last week, of which 23 were confirmed as coronavirus, up from 14 suspected in the first week of June, nine confirmed as coronavirus.
Sage documents released yesterday revealed that without an effective contact-tracing system, the return of all primary school children would push the reproductive rate of the virus above the crucial level of 1, even at current levels of social contact.
Even with contact tracing reaching 80 per cent of contacts in 48 hours — better than current performance — the return of secondary schools would be likely to push R above 1 once people return to half their normal level of work and leisure contacts.
It is thought that contact levels are just below this at present and that Sage has concluded there is hope of a full return by September if people interpret social-distancing advice very strictly.
If people increase their contacts scientists have warned ministers that they face the tough choice between keeping teenagers at home or shutting down other sectors of the economy to allow them to return to education. Pubs have been mentioned as an example of what might have to close."