The NASUWT teaching union urged schools not to penalise teachers who failed to return after the Easter holiday because of the suspension of flights.
Teachers and pupils returning from trips abroad have been caught up in the air chaos caused by a cloud of ash from an erupting Icelandic volcano and may not make it in today for the start of the summer term.
NASUWT general secretary Chris Keates said: "Most schools have arrangements in place to deal with this sort of emergency.
"The cause of the problem may be different on this occasion but the effect will be the same as when schools are hit by unexpected outbreaks of illness or severe weather conditions."
She said school leaders will have to determine if a school can stay open or if staff shortages are so severe that it is forced to close.
"Unfortunately, there will inevitably be some employers whose first thought is not focused on service delivery but on penalising teachers for failing to attend work," she added.
"Reports have already emerged that one authority in the West Midlands, Coventry, apparently has stated that teachers who fail to report to work will be docked pay and expected to reclaim it from their travel insurance.
"This situation is affecting workers and workplaces nationally and internationally. To single out teachers is totally unreasonable and unnecessary.
"Employers should be warned that they are vulnerable to a legal challenge for unlawful deduction of salary if they seek to penalise teachers in this way."
Cllr John Blundell, Coventry City Council's cabinet member for children, learning and young people, said he was not aware of such a policy and did not yet know how many teachers would be affected.
A spokeswoman for the Department for Children, Schools and Families said: "We are monitoring the situation closely.
"Schools have contingency plans in place and headteachers are best placed to decide how to cover for absent staff effectively and support pupils forced to miss classes catch-up quickly."Reuse content