Here is another fun way to make your students more proficient and simply to have a good time during the music lesson

If you have students who are already pretty good improvisors why not take it to a higher level and really test their abilities?

Use a playback software like Band In A Box to play a certain simple progression to your student

After a while, start changing styles every 16 bars.
do this untill they are able to move fluently between the changing playing styles

This is a very high energy excersize, on top of giving you very good indication of strong and weak styles for your student. Remember, the chord progression stays the same, the entire workout is based on affinity to different musical style like the differences between improvising blues, rock, country, jazz and so on.

This is also a great chance to sit down with your music student and go over the factors that make a style different from another style.
Anything from the type of scales used, chromatics, rythmic patterns and feel.

Keep Having a great time teacher music!

Jules