How about having the option to either 'pay as you go' or a one off payment at the beginning of the year?
I think some parents are fully able to pay for the trips but just forget others genuinely can't afford it.
I would prefer to pay £40 at the beginning of the year and then forget about it. If this lump sum were paid via the PTA (reg charity 1052715
) then they could benefit from gift aid.
I can appreciate that it is hard to predict an accurate amount at the beginning of the year but I'm sure that previous years should be a pretty good guide. Also the gift aid will be an extra 25% and the PTA bank is benefiting from the interest so if the school do 'guess' incorrectly at £40 they are 'buffered' up to to £50.