Have you found one yet? That might taper my answer but generally:
Firstly how is their knowledge? They will be expected to be able to tack up and do yard chores, are they capable of this without experienced help. I’ve got sharers for mine and while I don’t expect them to groom to a professional standard, I trust that they can skip out/ fill a hay net/ put a martingale on properly without needing me to check. Would they be able to ride by themselves sensibly (ie not just razzing around cantering the whole time)
Very very few yards allow unsupervised children so you’ll likely need to be there the whole time.
If you’re still looking be REALLY HONEST about their ability. If they’ve been in a riding school even for years in real terms they are still pretty novice. The average sweet PC/RC allrounder will probably be more forward and less forgiving than the more advanced horses at a riding school. They’ll also be doing a lot less work so generally more switched on and up for it.
Youll want insurance as well incase you damage the owners tack/pony kicks a car out hacking.
Be realistic about time commitments I’ve got three horses. Two have sharers. Apart from ensuring they’ll be kind, reliability is so important. It’s much more of a commitment than a lesson or two a week and a lot longer at the yard.
I would also say you need to be enquiring rather than your 14yo, most owners are wary of enquiries from young people as often they are well meaning, but dreaming and they haven’t checked with mum and dad yet! You enquring will be more serious.