I have a small bulletin board in my room (outside in the hall, actually) where I post these contests. If a student reads it and is interested, I will give them some guidance. If not, fine. I will mention the contests to them but I do not take time from my curriculum to work on them. The only exception to this is the red ribbon drug week,
which my school requires. I take one week for planning, we discuss aspects needed in a good design (this is after a good solid 6-8 weeks of design elements), etc. Then the following week we do the posters on 1/2 sheet of posterboard. If the kids want to add anything special that I don't have (like coloring in with markers) they can take it
home and finish it there.