As others have said, a private would, of course, solve your problems. But I can certainly appreciate the cost concerns. They are expensive!
Group lessons with groups larger than one will always have a difference ("split") in ability levels, interests, speeds, motivations, goals, learning styles, athleticism, and so on--no getting around it. Ski schools usually do try hard to make those differences not incompatible, and great instructors can deal better than most with "reasonable" splits. It is all skiing, after all, and really, the movements and techniques are fundamentally the same at every level of skill. Great instructors, however, are rare.
No matter what you do--private or group--you deserve to get a good lesson. As long as you believe your expectations are realistic, please express your concerns and disappointment to the ski school director, or a supervisor. They'll appreciate the feedback, and I don't know of a ski school that won't go out of its way to make up for an unsatisfactory lesson. At Keystone, we have often offered a private lesson with one of our top instructors to students who expressed honest and reasonable concern about a group lesson gone wrong. And they do go wrong sometimes, no matter who the instructor is, or where the lesson takes place. Please do yourself--and the ski school--a favor by giving them the opportunity to make it right!
It sounds like you are a serious student who is eager to improve, hellside. If you have not considered it, I cannot highly enough recommend our own EpicSki Academy. It is a greoup lesson experience--and I hear your concern about doing better one-on-one--but the groups are typically much smaller than most ski school groups, and the coaches--from across the country--are among the most experienced and dedicated in the world. It is the ideal program for serious students looking for some focused instruction in a multi-day program. You'll come out of it a much better skier, with plenty to continue to work on on your own. And I'm sure you'll have a great time, too, hanging out with like-minded skiers at some of the greatest resorts on earth! There's only one Academy remaining this season--at Big Sky, Montana at the end of this month. I don't know if there is still space available, or if the signup deadline has passed. But you can find out all the information by clicking on the link on the right side of this screen, or by clicking here.
Disclosure: I am one of the founders of the EpicSki Academy, and one of the current coaches. But don't let that stop you! Come join us in Big Sky!