It is a fucking stupid idea. Debra Kidd's blog outlines it very eloquently. Anyone arguing that it might work is deluded, and, I suspect, would be less in favour if their own child's school was involved.
I have worked with some excellent trainee teachers, doing PGCE placements, and also with some dire trainee teachers. It takes a hell of a lot of work, regardless of how good they are - being a subject or professional mentor involves a lot of paperwork, a lot of time spent doing one-to-one coaching and meeting, not to mention supporting emotionally - it's tough for 22 year olds, let alone 18 year olds.
This is clearly another hare-brained scheme to deal with the recruitment and retention issue but will only serve to fuel the "anyone can do it" mentality so many have already bought into.