I think the first requirement should be how many types of lines there are. It would be boring if all the lines are the same. The second requirement should be how much time and effort they put into their piece of art. If they just draw random lines and not try, they shouldn't be graded highly. The third requirement should be how much of the class period they spend working on the project compared to how much they spend talking. If they're talking all period and not working it wouldn't be a good use of time.