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.
That's taken from this page on the ATL site.