Pardon me while I look for my geezer-sage hat.

I've been teacher, student, and programmer. As a teacher, I don't always do exactly what my students think they want in exactly the way they think they want me to do things, and I am happy to listen to their suggestions and gripes. However, I must usually reply by asking them to trust me and humor me. They must trust that I am not being dictatorial for the pure love of abusing power and be willing to go along with he gag for a while. If they cannot give me that trust, they really need to go to another teacher who can inspire it. I've learned myself over and over that you sometimes cannot see the point of the teacher's wishes until you have had certain experiences yourself. Before I actually began teaching I was a good (but definitely not a passive) student with definite opinions who occasionally fought teachers in various subtle and nonsubtle ways. After I had actually taught for a while, my approach to studenting changed. I became increasingly willing to accede to my teacher's wishes whether or not I can see the point. It is an approach that has reaped me significant benefits over the years.

So, whether or not a particular bit of code needs commenting or not is not the real issue. You teacher thinks that commenting is a GOOD THING and wants you to get into the habit of doing it. I happen to agree in this case.

HOWEVER, all this being said, in my opinion the most important statement you make in your initial post is that "he had not mentioned that [commenting requirement] in the assessment criteria". If I am going to claim that a teacher has a right to expect you to do things his way for the duration of the student-teacher relationship, the teacher has a an equal DUTY to inform you, in detail, of his expectations. No informing you of the details has his expectations was not just a minor OOPS, it was a major faux pas. How can you hope to fulfill expectations you don't know about?