Exercising focus span -
1) Within a similar key, the student can be encouraged to play a minor scale on the RH while a major scale on the LH, (or vice versa).
OR
2) Complete different key scales (in major or minor) on each hand.
Personally I do find this method rather challenging but it proves to be very effective in increasing the focus span of individual.
While I find the importance of vast interaction between the teachers and students to be important, I've came up with the following methods for scales practices.
Exercising aural skills as well - Would be better if you have two pianos
1) The student should not be looking at the piano (if you are only teaching with 1 piano). The teacher will play any keys that he or she likes, staccato or legato, major or minor. The student will have to replay the exact scale.
Of course there are definitely room for improvisation, if you find the above method too dull. You could have the student replay the exact scale first (basic e.g. C major), then play the relative major or minor scales (e.g. A minor).
Speed with accuracy - Still better with two pianos
1) This is mere speed challenge while being really accurate in scales playing. Choosing of keys to play can be make interactive by the teachers or students' call (e.g Ab minor melodic, legato, left hand OR D minor harmonic, left hand, staccato, both hands). Each turn, shout a desired key and immediately after that, both teacher and student will have to play the scale at certain pre-set tempo.
A rather enjoyable, but fruitful scales practice session. Hope it helps!