I have several cartons of blocks I brought over specifically for these classes.
They're made of Japanese Linden--called Shina--and are easy to carve and print.
This weekend's class will have two block sizes of Shina plywood available; 8"x10" and 4"x6". The big block can be printed on both sides so 2 blocks will yield 4 surfaces (allowing up to 5 colors). The smaller block will make use of a floating kento--a jig that allows the whole block to be used and you'd be surprised how "big" an impact you can get from smallish blocks-- My "Fulcrum" print was printed from these small 4"x6" blocks.
The bigger blocks are easier--if more time consuming--to carve but allow for more detail since the scale is bigger. The smaller blocks are quicker to carve and allow for more layering/playing with the surfaces.
I'm hoping to teach the traditional method--a "black line" keyblock used to define the color areas of the other blocks--but direct carving and printing are also possible.