Well that explains much!
Lawrance Academy and Winchendon are very flexible with allowing “outside” sports. Winchendon is more flexible in that not only do they allow it, they encourage and facilitate it. However, you run into the same issue that Middlesex has in that the available hills are not known for producing top racers. Wachusett Mountian has an alpine team, but I have no idea if it is any good.
Lawrance and Winchendon are not top schools for academics, but like most boarding schools, ,they are what you make of them.
Deerfield has a strong ski team - and they seem to be very active, talented, and a friendly group. Holderness has a great program for students who ski. Kent Hill School (think they have their own ski run) Berkshire.