You can do a lot of decorating without using paint, and without having to wait for paint to dry.
There are many many sources of stickers, in lots of sizes, styles, colors, etc. You can also find peel-n-stick monocote products in many colors and patterns at your local hobby and R/C shops; a 6 x 36 sheet costs a few bucks and can be cut into dozens (hundreds?) of strips and rectangles. This is a great time of year to find colored markers in the back-to-school sections at the store. Colored markers work especially well if you are building with white body tubes, and absorb well into balsa fins. There are even colored vinyl tapes that can be used.
Not sure that I would recommend any of the colored duct tape products.
Anyway, long story short, you don't have to use paint. If you have a class with many rockets of the same design, it helps reduce confusion (at least, in the classes I have done) to leave one fin or a part of the body clear, to have a place to put student initials.
One way to use paint is to pre-paint the nose cones ahead of time, so you don't have to take up class time with it. If you use a variety of colors you can let the kids pick out their own; this usually makes most of the kids happy.
