Closure will "not generally be necessary" if schools experience Covid-19 outbreaks, according to official guidance for September school openings.
The Department for Education guidance published today says that two or more cases of Covid-19 in a fortnight may mean there's an outbreak.
But it says that if schools go on to take the recommended measures then full closures can be avoided.
"If schools have two or more confirmed cases within 14 days, or an overall rise in sickness absence where coronavirus (Covid-19) is suspected, they may have an outbreak, and must continue to work with their local health protection team who will be able to advise if additional action is required," the guidance reads.
"In some cases, health protection teams may recommend that a larger number of other pupils self-isolate at home as a precautionary measure – perhaps the whole site or year group.
"If schools are implementing controls from this list, addressing the risks they have identified and therefore reducing transmission risks, whole-school closure based on cases within the school will not generally be necessary, and should not be considered except on the advice of health protection teams.
"In consultation with the local Director of Public Health, where an outbreak in a school is confirmed, a mobile testing unit may be dispatched to test others who may have been in contact with the person who has tested positive.
"Testing will first focus on the person’s class, followed by their year group, then the whole school if necessary, in line with routine public health outbreak control practice."