As much as I like the idea of 200$ dev machines for students if they already have working laptops by far the easiest and uniform solution would be to distribute virtual box images, have students install virtual box and develop inside that, even that cheap Acer would be enough to run a ruby dev virtual box.
You can even install all ruby gems, packages and w/e you need for the class before you distribute it for convenience.
You can even install all ruby gems, packages and w/e you need for the class before you distribute it for convenience.