I've realised that the moment I have a writing assignment, rather than just an idea or two, or an inviting blank page in front of me, (yes, some of them actually are inviting) my flow of ideas dries up.
I think it's the syndrome that if something is optional, I can use it as yet another form of goofing off, so I get ideas about a novel set in a post-apocalyptic future when I am supposed to be designing a database. Tell me my job is to write a novel about a post- etc., though, and I start creating databases right left and centre, not a charred landscape in sight.
The way out is probably to allow myself lots of time to mull over ideas, and keep a notebook with me almost all the time to capture them. I am not going to be able to procrastinate and then cram for a creative writing course - especially not when I have ample opportunity to dither over a Greek course.
And some of the assignments! Describe something happening in a city street from the point of view of a child. Great, except (i) I grew up in the country, so none of my own memories are of cities, (ii) I was an only child and have no children myself, nor nieces nor nephews etc. in this country, so don't interact with children very often, (iii) I didn't really get to be a child myself, being more of a four-foot high carer for incapable adults than a kid. Apart from that, then, no problem.