Hello go_ and respondents. This tip might help with the F-sharp. I ask my students to play a (C) major scale very slowly and tell me what, in their opinion, is the most beautiful note of them all. A simple exercise, shove the notes down in order, cock the head towards the piano, listen to each sound and make a judgment. In almost every single case of this bold (because easily disprovable) experiment, they say that the B, the seventh, is the most gorgeous, followed by the G (5th). Try it. The seventh becomes beautiful for being so very nearly there, on the verge of arrival, but just not quite. The same answers come when the scale is descending, oddly enough.
So the scale, with no Swan-Lake changes in rhythm, becomes a melody, with event and even expression. One of the best tunes ever written.
Transpose all to a G-maj and an f-natch just doesn't have the niceness.
Also perhaps ask the student at what moment they "remember" that there's a black note coming. If they remember only on the note before, they're not giving themselves time to push the finger forward/upward that little bit needed to lift it from the white plains. A scale should be remembered as an entity, not a string of notes with annoying problems as to colour.
Incidentally, Chopin taught the B-major scale first to all his beginner students, this being the easiest to "find" on the keyboard and "cup" with the fingers. He only graduated them to C-maj once they had become more competent!