The role of a head teacher can vary quite significantly depending on which school they are working in.
A school may be without a permanent head teacher for a time but usually an acting head will be put in place for this duration. In an academy, there may be one head teacher across multiple schools so they may not be permanently based in one place. The job of the head teacher is to provide professional leadership for the school. They need to ensure that the correct procedures and guidelines are being adhered to and manage how the budget is to be effectively spent. When schools return in September, head teachers are going to have a very difficult job of managing the Covid-19 safety procedures that will need to be put in place to try and ensure that pupils and teachers are socially distancing as much as possible.
Some head teachers also teach classes, this is often the case in smaller schools that cannot afford to have too many additional teachers. The head teacher may be available for cover of a class such as when the teacher has PPA time or to cover sick leave.