How will schools that close know when it is safe to reopen?
That depends on the reason for closing. If an HPU advises that they should close (for exceptional reasons) they will also advise how long that closure should be for. During any closure, they would monitor the situation and advise the school when it is appropriate to reopen. The final decision to reopen lies with the employer, and is often delegated to the head teacher/setting leader. If the school closed for local school-specific (management) reasons, then it would be for the school to judge when those reasons no longer applied – for example, when sufficient staff were able to return from sick leave.