How many of the 9 patches have you made? If this is just the first one, ask your class instructor if she/he can see what you did wrong.
If you have done several, and they are all off-square, are the triangles for the four sides big enough for the longest side? You can sew them on, then square up the results, even if that means that they are no longer triangles!
Either way, don't drop out of the class!