Should you follow the rules?

Taught two very different classes yesterday, one after the other. In the first - a small group of non fiction writers who have been working together since September - one member was full of apologies for the work she was presenting. It was too short: 500 words when I asked for about 1000. It hadn't involved any research just chatting to her mother again, helping her to remember a time that's passed. It didn't include any of the figures of speech that she had learned in class....it wasn't what it was suppose to be.
It was much better. 
It was a simple, straightforward account of a simple straightforward act of a heriosim by a man none of us knew, in a place we had never heard of, for a cause we didn't believe in and it moved us all to silence. Written straight from the heart to the page, the writer followed the only rule that matters: she wrote. 
The other members of the group had - more or less - followed the exercise which was to focus on one word, research it and then integrate that research with writing of a more personal nature, family history or incidents from their own life. Each one was different. Each one worked, exploring areas of biography and memoir that the author hadn't considered before, may never have written about if they hadn't chosen that particular word and made new and exciting connections. 
I was so pleased. I was delighted. Hours later I walked out into the raw winter night  still feeling a warm glow. All down to me I said....all down to my brilliant exercise. Of course it wasn't, it was down to the writers and what they brought to it. How they made that original small idea entirely their own and in the process turned it into something bigger and better. 
As they trooped out of that London classroom, 20 students for my next class walked in. It's an introductory  class and even though it's Friday night they are a buzzy, lively group, eager to write and very eager to send in their first assignment.
There were questions about it - naturally. Concerns about what was wanted - naturally.
I hoped I answered them all but I have a feeling they would tell me if I hadn't. But one thing I did make clear was that this was creative writing and sometimes you look at an assignment with the best of intentions but your imagination springs to another idea and then another and you find you are writing at a tangent, off message, off the subject that you thought you were focusing on, you're off the page. 
And sometimes it works and it's magic. And sometimes it doesn't. Just like the original exercise you were set. But tangents are interesting places to be...
(NB if any of my university students are reading this remember that you are writing for a very specific and specialized readership - an academic board. So, let's discuss any tangents before you submit...)