Been there, done that! Sashing around the squares works really well to make the overall view of the blocks seem uniform. As you have it designed there are not any seams to match, so there shouldn't be to much of a problem.
Note: You may have to do a Quilt as you Go approach, in case your helper doesn't turn in her blocks until the night before the quilt is due! I had something like this happen, and I finally just tied the quilt at the last minute and had to turn in the quilt without all the binding attached (just pinned) because I ran out of time. The teacher didn't care, she was just happy to get the quilt.
Moral of the story: Don't Stress! Just plan for the worst and you'll be happy it doesn't happen like that. It will bring back fond memories in later years when you think back on what it took to make the quilt.