In my school we just use an excel based system that we set up ourselves.
Accountant (external to the school) gets all the material including the excel doc on disc at the end of the year so that he can audit accounts (not mandatory in our neck of the woods but we do it anyway).
I'd heard of problems with the early versions of Airgead Bunscoile but I thought the kinks had been ironed out in the later versions ... worth checking/asking about anyway.
To be Treasurer they'd have to be a member of the BOM ... a teachers' rep or a parents' rep or a Bishop's rep (if he is the Patron) or a rep of the community.