The short answer is: I don’t. I grade for effort, in a very coarse-grained way. If the student completes the project, following all the guidelines and requirements, they get full credit, regardless of the quality of the resulting music. (My assignment guidelines are always technical in nature; I don’t put any restrictions on musical style.) If students don’t follow the guidelines and requirements, or hand the assignment in late, or obviously half-ass it, I deduct points accordingly. I don’t give any consideration to the music itself when grading because then I’d just be grading on how closely the student’s musical taste is to mine, which would be arbitrary and unfair.
Okay, to really get your head around phase, we need to move beyond the on/off perspective we just outlined. Phase is quite literally a relationship of degrees. In other words, our “out of phase” sine wave has a 180° phase relationship to its mirrored counterpart. Were we to gently move our duplicate sine wave through all degree positions in the phase relationship, we’d encounter different degrees of cancellation, and therefore different degrees of amplification and attenuation — from silence to peak volume and everything in between.