I reviewed the score at work (where I can see it on the computer screen). I think the tune will work well in Dorian (raising the D flat to D natural), which you could accomplish either by adding natural signs where needed or by instructing the class to raise that tone. Regarding the dissonance, if you want to change it, you could do so by moving the B either up or down a whole tone (major 2nd) where you have it as the lower choosing note on the alto line at the fermata.
Chorale preludes are more or less standard components of the organ repertoire, and I have arranged at least a couple (Herzliebster Jesu with a text by Nathaniel Miles, Nun komm der Heiden Heiland) for the fasola community. I also minoried David Ivey's tune from the Denson book with his permission.
Randy