I think by 8 they should have a pretty good idea of the basic concepts of reproduction, as told to them by their parents.
I really don't understand why the school would be unhappy (or tbh what on earth it has got to do with them) about an 8yo knowing this information. I am sure the teacher already knows :)
I told my ds1 at 4, if he told others then so be it - although I don't think at that age it is the kind of thing they snigger about in the same way as 8yo's might (said ds1 is now 9 and is very matter of fact about it).
I also don't understand why you would be fobbing him off, if he wanted to know other things relating to the function of the human body - bones, bladder, heart etc - would you be fobbing him off?
Are you embarrassed about telling him about it? If so there are a number of books around which can explain the physical concepts, although I personally believe that it is important to combine the physical with the emotional and also talk about that.
Personally I would go with something like;
Babies are made when an egg from a woman joins with a sperm from a man, for this to happen they have sex - this is when people who love each other want to be very close and cuddle and touch each other, the man's willy goes hard (your 8yo undoubtedly already knows that happens!) and he puts it inside the woman's vagina.
I would also give some information about the female cycle and monthly menstruation (presumably he knows you bleed) and contraception.