The 18" is too short for your students to cut across the width of the fabric. I recommend the 6X24" for a first ruler, followed by the 12 1/2" square. After that, they can decide what rulers they might use the most.
I do NOT recommend the 6 1/2" or any long rulers with the extra 1/2". It's much too easy to cut from the wrong side and end up with incorrect widths. And since you are talking about beginners, they might not catch it until it has become a problem for them.
I prefer Omnigrid.