The death of a head teacher after testing positive for Covid-19 and the national lockdown message have prompted head teachers to seek clearer guidance.
The death of Cumbria primary school head Wendy Jacobs this week hit head teachers like a "bombshell", says the National Association of Head Teachers.
It is seeking clear detailed advice from government on how to keep staff and pupils safe in emergency schools.
It comes after the Prime Minister ordered a civil lockdown on Monday.
Mrs Jacobs, head of Roose Primary School in Barrow-in-Furness, died on Monday after being admitted to intensive care with coronavirus symptoms.
An NAHT spokesman said it was "really mindful of individual cases", but the death of Mrs Jacobs had really hit the profession "like a bombshell".
Chair of governors at Roose Community Primary, Fred Chatfield said: "This is devastating news for our school and nursery community and all our thoughts and sympathises are with her her family."
In recent days, school leaders and their teams have worked hard to patch together local solutions to the national problem, by remaining open for the children of key workers.
It is thought the vast majority of schools opened yesterday offering reduced provision, with a survey suggesting most schools opened with fewer than a quarter of staff present.
NAHT general secretary Paul Whiteman wrote to all members this morning, saying: "Having heard the Prime Minister speak, many colleagues will be rightly concerned for loved ones and understandably anxious about the personal risk of simply going to work.
"It would not be surprising to find, in the absence of clear government advice on keeping safe, that fewer colleagues are willing or able to work today than yesterday."
School staff know that they are taking a risk by reporting to work, he said, against advice given to the rest of the population to stay at home.
Many will be caring for the children of frontline workers, which the latest scientific advice suggests may spread coronavirus.
"It is only right that this is a choice informed by hard evidence regarding the degree of risk involved," Mr Whiteman said.
Mr Whiteman added: "We need a step change in the detail of advice given to schools now. Our focus will be to reiterate the need for schools to be given clear guidance on how to keep pupils and staff safe.
"It is absolutely vital that we have sight of the expert medical evidence on safe levels of attendance and density, and are provided with practical advice on the protective steps we should be taking in schools."
He added: "From masks or other PPE, to distancing children from each other, to sufficient supplies of soap and hand sanitiser, schools urgently need answers to their questions about effective safety measures."