The union has now written to secretary of state for education Gavin Williamson urging him to allow all school staff to be allowed to wear face coverings if they choose.
UNISON also supports calls to let pupils wear face coverings too, given emerging evidence that transmission rates among secondary school children could be equal to those for adults.
Employers must also provide safety kit, such as masks where necessary, to staff found to be at higher risk of Covid-19 following an individual assessment, says UNISON.
It comes ahead of updated government guidance coming into effect this weekend which makes face covering compulsory for all inside public spaces from Saturday.
Current Department for Education guidance to schools needs an immediate review to ensure measures to protect staff, pupils and the wider community are improved, says the union.
UNISON senior national officer for education Ruth Levin said: “Parents, staff and the public are suffering needless anxiety because of the government’s confused messages around face coverings.
“We’re just weeks away from a return to the classroom and staff and pupils need to know where they stand. UNISON believes they should be allowed to wear face coverings if they want to.
“It’s been a shambles from the start on what the rules are. The government must act urgently to sort out this mess and ensure the reopening of schools is as safe as possible for staff, pupils and the public.”