DH has a close knit community as do all cycling disciplines, everyone looks after everyone
You will be keeing good company
You need to be alert, fit healthy to do it well
You can’t waste money on drugs and booze as you are funding your bike, ride kit etc
Being younger it is better to learn now than when older
Discuss (you need to get them) stats on injuries with Football, Rugby, Soccer, Basketball, Squash etc, their injury stats are pretty high, in particular knee injuries and head injuries, with DH you have safety gear mitigating risk
Discuss driving injuries with the young, in particular hoon ‘race car’ drivers who fang on the roads and not race tracks
Discuss the element or bad element near your home area, tell them there is pressure for you to join a gang which you don’t want to do, (you’re a good kid remember)
Discuss how you could be run over by a car on any given day, give them stats on the road toll, accidents etc
When you are better go back to DH, get your friends to video you and show your parents how you can actually do it and how much joy you get from it
Agree to pay for all of your bike requirements
Offer to help more around the house (give and take)
Promise that you will try harder at school (that usually turns parents on and will actually hep you heaps in later life)
Tell them if you don’t ride you can see yourself losing your friends network, which is most important as it keeps you going, discuss depression rates in the young who could have been helped by having a focus or a group of friends to support them with a common interest
Tell them it is not as if you are going to hurt yourself every single ride, discuss why you stacked, there is usually a reason you can pin point, Conditions? Ability did not match aim? More training required? Parts failure? You get the point, show them you have learnt and know why things happened.
Remember, don’t say to your parents you don’t really care, you just want to ride DH no matter what, that is a stupid argument in anyone’s book and you will lose here. Tell them what you have learnt from the stack, how you will do things differently to reduce the risk further. If you stacked and don’t know why or are a balls out c*ck out rider with only that and no skills, then you will come a-cropper again and it is probably best you choose something else. But it sounds like you are better than that, just struggling how to discuss it with your parents, always remember they really give a 5hit about you or they would not be challenging you.
Finally, you say DH is the only thing you are good at, I reckon your parents think/hope/believe that you may apply yourself to other things as well, the way you do to DH so that you can see what they see, which is you are or can be good at many things. So don’t just shut your parents down, discuss things calmly with them, make them aware of risks in everything we do, discuss the benefits of DH but most of all the value it gives you and the value it can be for them too if you end up agreeing and perhaps applying yourself to many things, not just DH. Work with your parents.
Good luck.