I have to say, I'm not sure I agree with all of that. I got average to mediocre grades in fifth year (mostly highers, but mostly Cs), mostly due to a lack of maturity and commitment as you say. I got much better grades in sixth year, but my grades at uni were hugely better than any I got at school. In my view, people mature at different times and are best suited to being taught in different ways.
Some people bloom when they get to uni, others do badly at school, but then go on to make a much better fist of things a few years down the line in college, and then go on to do well at uni. So while I agree with the first part, I don't really agree with the second part. Plus those that get top grades in school rarely seem to (in my anecdotal experience this time) do nearly as well at uni.