Write what your students write. When they are doing an exercise that you have asked them to do, the teacher can do the exercise also. This will give you a feel for how long the exercise will take, remind you of how easy or difficult it is, and give you things to point out when everyone is done.
ALSO. Save those exercises. They might turn into something good. What if, as an exercise, you had them write a first page of a novel. What if you brought in some first pages from some excellent contemporary novels and passed them around and talked about them? Who knows? You might get a novel out of it! (If you can ever find the time)
I had my students, in a 3 hour graduate class, write microfictions. It's hard to fill up a three hour class when they have not read any textbook assignments. Of course the great thing about creative writing is you can vary lecture, discussion, writing exercises, reading out loud (their exercises or an assignment), and workshop. One thing I had them do was write a microfiction. What I created doing the exercise myself, I wasn't sure if it was a prose poem or a microfiction. If I couldn't be sure that I was making what I'd asked them to make for the first time, then I decided I should tell them not to worry about what they came up with.
I went back to the office, and over the course of the week, played around with my exercise, and I think came up with a half-decent microfiction after all. One poet I know admitted that she gets many poems from the exercises she does in her own classes. Some exercises are designed to focus on one thing specific, like dialogue. Maybe that exercise will not lead to anything for you, but maybe sometimes you write stories that are almost all dialogue. I do.
It's a bit of a dirty secret, that a mere exercise could lead to a published poem, or story, or essay. But why not? We really don't get much time to write with our jobs, so use your exercises, for yourself and for your students. Don't let them get lost. I did a haiku up on the board about kids staring delighted at neons in a fish tank in a department store. It wasn't bad, but I erased it.