Designing a curriculum from scratch is tough. If you only had to make lesson plans from a given curriculum, that would be much easier. (The following is one style for teaching creative writing. There are many other approaches you could take.)
The first thing you want to focus on is deciding what forms of creative writing you wish to teach. Just prose? Prose plus poetry? Creative non-fiction? Most general creative writing classes do poetry and short fiction, with maybe some short-form creative non-fiction. Once you decide what forms you want to address you need to decide how much of the class is going to be devoted to those forms.
(You might also want to consider whether you will be approaching this by using specific writing forms as the starting point, or whether you might want to include some general instruction of stuff like similes and metaphors, or character arcs, or poetic devices.)
Once you have your general curriculum together, you should consider what form each class period will take. How much time do you have? Do you want to do a writing-prompt type thing as a warm-up? That's a pretty popular tactic in intro to creative writing classes from middle school on through undergrad. Some people use prompt books for it, of which there must be at least a million or so, I imagine.
If you think the kids are relatively decent writers already, you may not even need very much supplementary lecture on anything and can just let them do assignments, perhaps with a focus on a given writing technique. If not, some writing exercises focused on various techniques could be useful.
You'll probably be doing both teacher critique and peer-critiquing. Group critiquing among students is pretty popular. You might want a rubric for a high school class to give them some idea of what can be useful. You might want to limit peer critique to a few large assignments, and in between have smaller assignments which you give feedback on as the teacher.
Going by your description of the students, I would suggest teaching the basics of creative writing and having only two major assignments. Probably a poetry one and a prose fiction one. You could substitute creative non-fiction for either of those if you feel the students would like that more. Writing a narrative about/inspire by something that's happened in a student's life is often a fairly popular assignment and gives them some emotional investment in the course. You'll probably want four to six minor composition assignments, depending on the exact number of class periods.
I'd definitely suggest 5-15 minutes of free-writing at the beginning of each class, depending on how much class-time there is. You can use a prompt book, give your own prompts, or just let them write whatever. It should be a writing journal as mentioned by a poster above. It would give you some idea of how the students are improving as individuals and what spots they may be most lacking in. It's also very low-pressure, consistent writing practice, which is good for students, especially at these ages and with these experience levels.
If you've taught more standard English classes before, you could use one of those curriculum plans as a base-line skeleton, possibly.
