Although to add that not every parent has a dead set "career path" for their child and it is OK to send your child to a school with a wide range of next schools.
For example, I have two DDs at a co-ed prep which goes to 13. Girls leave at 11 and mostly go to grammars and local independents (a real mix of top academic schools through to ones known more for pastoral care). The boys go to a mix of independent day and boarding schools. At both 11 and 13 some children go into the state sector.
We purposely chose our school as it kept our options open. Whilst we have every intention of staying in the independent sector my husband's job security is often volatile. In the same way that someone might choose a school based on very specific next schools (Eton, Harrow etc.), some want to wait and see what happens and have no definite plans until the later years of their child's prep school "career". I believe a good prep should work with you to identify which next schools are best fit for your child. If they are not Wycombe Abbey material then they should be the voice of reason.
It is therefore so important that you do think about what you want for next schools. Aiming high then choose a school that consistently sends several children each year to the schools you like the sound of. Are you focussed on grammar schools and therefore need an 11+ factory instead? Are you thinking boarding or day school? Or are you like us and it's only from Y3/4 onwards that it has actually become clear what sort of school we need, but we know our school can help us with that?
I agree with the poster above about parents getting grumpy when the school doesn't match expectations. Our school likes the boys to stay until 11 and so the 11+ prep for them isn't as strong as it could be. The school is very open about this though.
This is one of my local single sex schools listings of destination schools as an example:
www.lockerspark.herts.sch.uk/about-us/destination-schools-2/