Financial compensation is being demanded for teachers and other staff left unable to work as a result of catching Covid-19 in schools.
The NASUWT also wants "more supportive sickness policies that recognise the complex and changing nature of long Covid symptoms, which can see teachers going through extended cycles of illness, leading to periods of intermittent absence over a long period".
The teaching union is worried about latest Office for National Statistics (ONS) data which shows that those working in education had the second-highest prevalence rate for long Covid last month at 114,000.
The figure - for the four weeks ending 6 March - was only just behind healthcare workers which had the highest rate with 122,000 cases.
NASUWT general secretary Patrick Roach described the figures as "deeply concerning".
“The number of teachers who may be affected with Covid-related long-term illness could be a ticking time-bomb," he said.
“Teachers need to be supported by sickness management policies that are compassionate and recognise the complex and debilitating nature of long Covid. Teachers experiencing long Covid should not be made to live in fear of losing their jobs or of financial hardship if they are no longer able to do their jobs."
The union says improved access to ill-health retirement must be given to provide financial protection for teachers who are too ill to work due to the impact of Covid.
And Dr Roach added: “Ministers must, as a matter of urgency, provide financial compensation for all teachers, including supply teachers, where their careers have been impacted due to Covid-19.”