Agree with the binding suggestion and the squaring up. I can square up the small blocks but the large ones still get to me. Borders are important to take the three measurements (top bottom middle).
The tough thing about teaching that I have found is that you often have opposites in class. the one who is way way too nutty about being exact (important to be precise but don't go crazy) and the other one who says, "Oh you know me, I don't want to worry about being exact. Close is good enough." Hard to balance those two out. The exact one gets hung up and the 'not-to-worry' one can't finish because nothing fits.
Good luck.
Dot in Oregon