Paint with a heavy body is too expensive to buy for a whole class. I tried mixing paints with plaster which was too brittle. Flour was bad, and colors with both were far too pastel. Then I thought, "why not paint with texture first, then paint over that?!" It seemed to work, and I was able to really focus on texture as an art element. We used brushes to apply the acrylic paste dispensed on paper plates. I cut popsicle sticks to a wedge point on one end to use as a pseudo-pallet knife. I worried that the kids would use too much paste and I might not have enough, but most used a tablespoon or two on our 6 x 8 in. canvases.
I have a collection of images for students to choose from and our motivation was to pick an image that would be the beautiful view you'd like out of your vacation home window. This way they had a broad choice. I could also see having students look up landscapes or landmarks from their own cultural backgrounds, researched and printed out as references. Whatever they use, there should be a way to make a personal tie-in so that all have a reference that means something to each individual rather than forcing them to paint what you have chosen for them. This could also work as a plein air impressionism project should you be able to take your class outdoors.
The examples below were by students ranging in age of 7 through 16 at my summer art camp program. This will definitely be something I will do in my school classroom this coming year.