It really is a case by case basis, but more than likely if you tore your ACL (fully or partially) and want to ski or compete at a high level without fear of further injury to your knee, you're better off getting surgery by a very well respected Orthopedic Sports surgeon (that performs ACL surgery often), and rehab with a good physical therapist.
The twisting, edge pressure, high degree of stress or torque on the knee are so great when ski racing, skiing bumps, hitting jumps, going hard on ice or groomers, jump turning on steeps or backcountry skiing, that any bad ski fall, knee twist, or force can lead to further injury.
If you're young, chances of re-injury and doing even worse damage to your knee joints is a real good possibility if you want to continue to ski aggressively without having surgery to fix the torn ACL.
That being said, there are some people who are able to ski and even ski race at a high level with a torn ACL. But to do so you really need to wear a really good knee brace while skiing with a torn ACL or else you risk further damage to other ligaments in your knee.
Here's a link to a good read that has comments from people who have torn their ACL skiing and what they did. Keep in mind, it's cross-country skiing related, but a lot of people tore their ACL downhill skiing.
Torn ACL advice here: https://skipost.wordpress.com/2011/06/01/torn-acl-advice/