Thank you all for your input. I have narrowed my choice down to the 12 1/2 inch square and the 6 by 18 inch rectangle. But which one should go first if someone can only buy one?

If you can, get the 6 x 24 rectangle because that one is long enough to cut the folded width of fabrics up to 48 inches wide.

Again, it depends on what you are planning to do.

I cut most of my strips on the lengthwise grain - I don't remember the quilter/author that I learned that from - that way at least two edges are more or less on grain.

If you are planning to cut strips from selvage to selvage (crosswise), definitely get the 24 inch ruler.

My opinion.

I think I started with a 6x24" but soon decided I needed a 6" square & then a 12" square etc. I would add them to my collection when they came on sale.

The first ruler I bought several years ago was 6 X 12. I only recently bought new different size rulers.

i would go with the smaller size ruler first since we cut so many small pieces. It's so hard to cut 2 1/2 in. squares with a 12 1/2 in sq ruler!

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.

i have all sizes i have a bad shoulder so i just brought one with a lip on it i love it ,it really helps me for cutting stright i just anchor it to the edge of cutting mat and i don,t have to move my hand up so much it helps