You should NOT cover for striking colleagues.
This undermines their democratic right to withdraw their labour.
Please support colleagues on strike by following the official advice of the teaching unions.
NASUWT official advice
NASUWT members should:
make clear to the headteacher/principal that they will be reporting for work as normal;
make clear that they will not accept any variation to their contracted duties and/or undertake the timetabled or other responsibilities of those engaged in action, including taking into their timetabled lessons pupils from classes of teachers who are involved in strike.
That's taken from this page on the NASUWT site.
ATL offical advice
ATL members should advise the head teacher that they are available to work normally, but they are not willing to accept arrangements which undermine the industrial action of colleagues. This is normally done via the rep.
ATL will normally consider it unreasonable for you to be asked:
- to take over the work of colleagues engaged in industrial action, other than in exceptional circumstances (such as genuine emergency)
- to undertake a teaching load greater than usual or to accept additional responsibilities or duties as a result of colleagues taking industrial action
- to agree to the amalgamation of groups of pupils or students or to the division of one group between others as a result of colleagues taking industrial action.
That's taken from this page on the ATL site.