CarrieBlue I'd never been to a state school when I started my school based teacher training in an inner London school. I went to an English private school but had 2 years of experience of teaching secondary school classes, though all oversees EFL teaching.
I think tbh it will have been the arrogance and lack of experience teaching and with children/ teens generally.
Being fluent in a language (or having a PhD in pure mathematics, or being a sought after software engineer) does not by any stretch of the imagination mean that you'll be good at teaching MFL/ maths/ IT at GCSE level.
Obviously they can't expect an experienced teacher, but 3 days experience is nothing - my DD did longer on her year 8 work experience placement!
You can't know you want to teach after 3 days.
I'd say substantial experience with the age group would be a reasonable expectation, even in a non school volunteer context. That's for the applicant's sake too - how do you know that you want to work with teens Needadoughnut ?
It also doesn't sound as though Needadoughnut sold herself well - you have to be ready to say why everything you've done has led you to where you are today, on the verge of what you've come to realise is your vocation as a GCSE MFL teacher!
It'll be BS to a degree, but interviews are a game... 