I agree with what Tray Surfer has said.
There is a bit of training available, such as college courses after school which some people elect to attend now that I believe you have to stay on in some form of further education for a year after your GCSEs.
I think a combined training day between flight deck and cabin crew would be extremely valuable as a current member of cabin crew, however it will be of little to no value before you have the job as people would not appreciate it nor be able to relate to the importance of it.
I would say close to 99% of cabin crew that I work with are unaware of what PPRUNE is.