I still remember a large mural my own class made with this technique when I was around this age - approximately 30 years ago now! : o
It was super exciting for me and I'm pretty sure these students were equally as excited. I think our class made stars and planets back then, but this lesson was inspired by a Mambo t-shirt of mine featuring some kind of electric guitar playing, rock 'n' roll alien. It was descending from a flying saucer with the caption "Planet Rock!" So I showed this year one class my much loved and well worn shirt and we also looked at some other UFO illustrations.
We spent a full lesson with each student covering their paper in flouro oil pastels - just scribbling thickly everywhere with no deliberate structure. Then they completely painted over this twice with black acrylic paint. Once these were dry everyone was armed with a toothpick and then scratch drew their own UFO scene. It's worth noting the friendly looking aliens they have imagined : )