Depends what you call "high altitude". Specific training is still available in Canada I think. No ideas about the US.
As for "ab initio" training at high altitude it would take a while to notice the subtleties required. On the other hand I doubt you will find a school that operates training types at high altitudes. High altitude flight in a helicopter can be difficult and continues to catch some very experienced people out. It's possibly not the altitude itself it's just all the other things that can come with it like snow, whiteout ( very dangerous and highly misunderstood by most), wind, slopes, sling loads etc.