I think prep schools with very small year groups tend to be the cheaper ones. That is a key factor why people pick them over the larger, more expensive ones.
Assuming that the larger prep schools with 2 or more class intakes also have class sizes under 20, what benefit of a small year group of less than 20 in a co-ed school for a child at the top of the school, whether that is Year 5 and 6 or 7 and 8 (and most of those tiny Preps finish at 11 - few small ones go all the way to 13 and prepare pupils for entry to public schools which start at 13 usually) is there?
Unless your child has special needs of some description, I honestly cannot see what benefits a tiny school can deliver that a much larger one which still teaches it's bigger year groups in small classes cannot deliver, in terms of curriculum, facilities, extra curricular, quantity and breadth of staff, friendship options, preparation for a range of secondaries etc. However, I can see loads of things a very small school cannot deliver in all of these areas.
Yes a small year group with one small class delivers a small class teaching environment. However, this is only one aspect of education. People pay for having specialist teachers for all subjects - does a tiny prep provide a different, specialist teacher for each of Maths, History, Geography, French, Latin, RS, English, Drama, Music, Games, Swimming, Art etc etc......I doubt it, because they simply won't have enough teachers on the staff to allow for this. People also pay for extra curricular range and facilities - will the tiny prep be able to field teams for weekly matches and tournaments against other schools in a range of winter and summer sports? Will they be able to provide specialist coaching, holiday coaching courses and quality facilities? Will they be able to get out a couple of orchestras - one for younger kids and one for the older ones, a couple of choirs, ensembles for strings, woodwind etc? Will they be able to group students into large enough groups of the same ability to ensure effective differentiation, but also a decent number of kids at the same level to bounce off and provide a teacher for each group, not just one teacher who rotates round tables? Will they be able to diffuse friendship issues if needed by missing classes up and cope with two or three of the same gender leaving the year, without it wrecking the gender balance?
It just seems to me that many of the tiny prep schools are not worth paying for. The limitations that tiny imposes or the risks at least are significant.....to the point that unless a child is going there because of SEN or social difficulties, I cannot really see what is being gained by paying, unless people take the view that simply by paying, the school must be better, which is a bit daft. Perhaps I'm missing something crucial.