Actually it could well be worse than GBH for the villain. If the victim is 15 and the perp is an adult then that's an offence against a minor - jail time and child offence register likely.
That would, of course be a gravity factor in any decision to prosecute and I would hope that such an attack would attract jail time but it's certainly not a given.
Capetonian, this isn't the seventies you know. Nobody falls down the stairs anymore. Mores the pity I often feel.