I don't see the problem from learning from mark schemes - it is an excellent way to make sure there aren't holes in your knowledge.
Anyway I feel the people claiming A Levels are ALL about exams, you don't learn anything, etc, are kidding themselves for the sake of being contrarian. I have learnt a lot in the last two years, and the work I have done has been excellent preparation for the degree I want to do (PPE), as well as reading more widely via english. If you can't see any merit in A Levels, the problem is with you.