With 18, I would suggest the simpler-the-better. I did a project with nine 3rd graders and forgot to get my 'squeal' immunization ahead of time . Before starting on a project, the girls were shown the mixers, the wedging table, the wheel, drying area, glaze containers, and kilns...just as a walk-thru on the steps to make a pot. My favorite question, "How come you can't do it on the breakfast table and just pop it in the oven?"
In preparation for the 'nine', I made 6" tiles, laid them out into a 3x3 square. On a sheet of newsprint the size of the 3x3 tile lay out, I sketched out a globe with a rough outline of all the continents, then cut it out into 6" squares. The girls each took a tile and a piece of the 'puzzle' and were shown how to transfer the design to the tile, add texture, stamping, loop-cutting, and adding tiny bead-balls to cover the line work of the design. They were not told ahead of time that the puzzle pieces worked together. It took most of the girls about 30-40 minutes (with some reminding of their leader that it wasn't a race). Once finished, I then assembled the tile puzzle in front of them with about a 5 minute object lesson on how working together can make the world a better place...that's where the squeel happened when they saw they had created a 'world'.
The next 30 minutes was all 'pinch pot', using two colors of clay and their newly acquired texturing skills to decorate their pots. I think all of them had made a pinch pot before. I pulled out a tray of letter-type stamps, and using those on the pinch pots took the remainder of the time.
Just a few of the girls returned after the bisque firing and added stains and glaze (that worked out very well). The tiles were then glaze-fired, assembled on a board, framed, and presented to the leader at Christmas...'wish I had a picture, it worked out pretty well.
Best of luck with your project!,