The F in the 11th bar is in harsh relation with the F# in the 13th bar because one intervening note (I guess it's the G, since you've said it goes up) is simply not enough for the ears to forget. Moreover, F is a "fa" note while F# is a "mi" note producing the "mi against fa" effect, which considered bad in that time, both melodically and harmonically.
So this is bad:
F G F# G
If you lead F (fa) down, the situation is better, but still bad:
F E F# G
The solution of the student is ok, but still produces a poor melody I think:
F# G F# G
Try singing(!) your countermelody and you will know if it is melodically bad or good.
About the 10th->8ve, it's simply a false cadence, I think, because it mimics a 3rd->unison cadence at an octave distance. You can avoid using it in 2 parts. The best to approach a 8ve is oblique motion or contrary motion from the 6th.
Máté