I have one with a lip - and I never use it. It annoys me.
What are the people going to be doing in the class? That might help determine what ruler to use.
A 6 or 6 1/2 by 24 is probably the most useful if one can only have one.
I think the Omnigrid ones are the most accurate.
For cutting little pieces, I LOVE my 4 x 8 one with the 1/8 inch grid.