School Voluntary Fund

I don't do these, however I assume you are talking about PTA raised funds, or other types of money raised outside of the school's normal budget.
If it were me, I would just try and make a judgement call about the value you feel it is worth, considering the amount of information you need to go through, and contrasting that against the fact that the more you charge the less they have to do anything useful with the intended audience!
I'm also a commissioner for Girl Guiding, and a few of the units I oversee go to other accountants to get their books assessed. Not many actually charge for this, where the leaders are already clients with businesses.
Not sure if this really helps, but I thought it may be food for thought.