How is it not appropriate for someone who knows quite a lot about some curriculum areas and is prepared to help for free to pass that knowledge on?
I give six hours a week unpaid to DD's school. I'm not a fucking saint. I'm a mum with good knowledge of most core curriculum areas (please, don't make me teach humanities, I have no idea about those) who likes helping and is good at it. When I go in to help, most of my daughter's class beg to come and be in my group. They know that I like them and will make it fun. The teachers know that I will actually help, in terms of adding value to whatever they are currently teaching. And I am fortunate enough to be able to earn a living and still have this free time.
Last year, in Y2, I took a regular group of underachieving children for Maths, specifically word problems. I did a half hour session once a week with six kids (half an hour personally for each child). Five of them achieved Level 3 at the end of the year, although at the start of the year they were all counted as being children who weren't achieving their potential. I am very happy to have made a difference to bright kids whose home lives meant that they weren't achieving their potential.
This year, I'm doing other things, mainly with children who are struggling with reading. I hope I'll be able to make a difference this year too, and have seen some progress already. If I don't make a difference, it won't be because this is somehow massively inappropriate. Quite honestly, most of the stuff that my hugely overachieving DD can do is stuff that I've taught her, not stuff that has been covered in school.
I think I am glad my DD doesn't go to your school, TheTroubleWithAngels.