That's a very good question, which I sought an answer to quite incessantly over the last year.
My son is a teenager, he was applying to a sixth form.
As I see it now, my son would have benefited from:
More social interaction with peers and relatives, and more rough and adventurous playing, and more contact sports.
More team sports activities, and in general more sports, particularly in groups.
Ideally more interactions with alternative, better role models than us, as parents.
Activities such as scoutism or similar, which boost self reliance and resilience.
Having siblings, and having more friends and close family.
Mind you that if I had been asked a few years ago how my son was doing, I would have said well enough, in his own way.
Since very young he wrote stories and comics, had a sharp sense of humour, composed music and songs, made videogames and was a bafta finalist with one of them.
He said he did not like sports, but did swimming for years and is quite proficient.
He could be timid but was quite communicative once he knew someone a bit.
And of course he had very high grades, great behaviour and was very punctual with attendance and homework, without any input from us.
He was lonely in school, I suspected, and I hoped that will change with time, because that's what happened to me at his age. But I had way more opportunities to make friends and go out with them, and many cousins I spent summers with.
When it all came down crashing he became extremely silent and forever absorbed in his own thoughts and worries. He suffered from panic attacks, and explosions of rage, and very paranoid thoughts.
I see this as the result of isolation mainly, to an extent of bullying, which we found out about quite recently, and of a quite unhealthy perfectionism which I might well have inspired.
In the end, I have no useful advice to offer to others, I think, because it is all quite specific to our situation.
We are foreigners and we chose to live in the UK mainly for professional reasons. We made friends of course, but in London people come and go, and we have no relatives here. So we never gave him a social network and other significant adults most kids naturally have, and we did not put sufficient effort into offsetting that by making new friends and involving him in social and group activities.
We have been very loving to him, and very present and available. But kids need other kids, and family, and it was quite dumb of us to underestimate that.