Here's a super easy lesson that anyone can use in their classroom. It is appropriate for almost any grade level. I did this at my home with very young children.
First, they painted tagboard with acrylic paint. They just free painted and covered the entire paper. Then, they drew a vase on newspaper. They drew/colored patterns with crayons on the vase. They cut out the vase and painted it with pearlescent watercolors. Then, they cut out the center and petals for the flowers from that painted tagboard. (Obviously, the tagboard had been left to dry overnight.) They used green construction paper for the stem. We chose a blue background and just used long construction paper. Another option would be to have them use cool colors to paint a piece of tagboard for the background. Then, they could use warm colors for the flowers and neutral colors for the vase.