One thing we have found as a family is that the "ski vacation" is a great way to develop skills in the kids. We have a group of families that take a week at Lake Norris in TN and we all bring our ski boats (This year we will have 4 slalom boats and 1 wakeboard boat). While this is not a ski school setting, it gives a ton of water time which is often the biggest need as it pertains to ski development. We have done this trip for almost 8 years and watch kids go from total beginners to learning to barefoot, slalom, basic trick skiing, other goofy things. Last year we invested in a portable course and dropped that in. The kids really advanced having ski sets whenever they wanted them. Of course, with one of these big lakes, you need to find a spot to put the course that has a decent shoreline and minimal traffic which is easier said than done. The east end of Norris is dramatically better than the west as it pertains to boat traffic.
Obviously, if you are seeking professional coaching this type of vacation does not work unless you get someone to travel to you but thought I would throw it out there as a brainstorm.