One common mistake I warn students about (especially art and audio students, and to a lesser extent game design students) is to never include substandard work in a portfolio just to show how much you've improved. Game companies don't care about how much you've improved, they care about how good you are now, and whether you can help them make a great game now. If you put mediocre work in your portfolio, the message you send is that this is the best you can do.
It occurred to me the other day that this might not be the case for teachers. I've never heard of an instructor putting together a portfolio of their own students' work to show how much their students have improved under their tutelage, but I don't see why something like that wouldn't be valuable if you're marketing yourself as a first-rate teacher.
Likewise, a university might consider this for its promotional materials, the same way that the beauty industry likes to show lots of before/after photos so you can see how much of a change their products can make. Again, I've never seen this before, but at the moment I'm having a hard time thinking why not.