My school actually had standard coding styles for each language they taught. Naturally, they all wanted a metric fuckton of comments (luckily the TAs didn't care as much about the comments part of those guides).
Particularly with beginners, I think there's a lot of "What were they thinking when they wrote this??" going on when a professor or TA reads code that is completely unintuitive (Take a look at /r/learnprogramming for some examples). Having comments everywhere makes it easy to say, "Oh! S/he was trying to reset all of the variables in the program by calling main(null); (actual example from the subreddit). How would you ever know that without a comment explaining why main(null); is called in the middle of a class?