for my machine quilting class, we were told to bring a yard of muslin, but instead i took some ugly solid green broadcloth that i wanted to use up. i took the muslin, too, just in case. turns out the green fabric was just fine (actually showed my stitching nicely) and i've kept my 'first designs' as a keepsake!
if you take both, you can always use the muslin later for a project (backing for a wallhanging maybe?). you'd just have to cut it up into the squares quickly if she insists you use it (if you can't get ahold of her, maybe go a few minutes early to take care of details).
in our class, we used the yard and divided it up (with marked pencil) into six sections. that way we were working with a larger piece of fabric to get used to manipulating it on/in the machine bed.
i would definitely go with a solid, though. you'll want to be able to clearly see your stitches and any tension issues you may have.
what thread is she having your bring?