I bought this little book at Barnes and Noble the other day for the workshop I am co-presenting. I think with some tweaking, you can figure out ideas for topics and or just use what they have with your students. Each page has a quote and then a writing prompt.
Good luck as you prepare for those tests...ARGH!
One sample is...
"Always do your best. What you plant now, you will harvest later." -Og Mandino
What do you hope to harvest through hard work and dedication?
How about an expository idea...
Explain why it is important to work hard.