I work in a large secondary as a member of support staff. The business manager is expected to do everything.
She is responsible for all HR, head of finance, manager for all technical support staff, health and safety lead ( remember schools hold chemicals and radioactives, so has legal responsibility for this)
Overall responsibility for caretakers and school upkeep, as well as managing the lettings of school buildings.
Ours also has to do duties ( playground and dinner hall) and does a couple of stints in the removal room ( naughty kid room)
When there is a governor meeting, which is typically after school 6pm she will be in attendance.
SLT meetings are always 5 pm till miminum 6 pm midweek each week.
All SLT are on a 365-day contact at our school. Ie in an emergency she could be required to work every day, even weekends, until over.
Oh and don't forget the caveat in every support staff contract. Any other duties as required.
She works hard, and I would say even though she gets a good wage, it has to be the hardest job in the school.
If you have worked in a school before and are going in with wide open eyes , go for it.