Improvising need not be restricted to the 12 bar blues.  ICT can be used very effectively as a backing for class improvising.  Load the MIDI files into a sequencer and play them as pupils use the notes of each of the modes for improvising.
If necessary, reduce the number of notes for less able pupils.
 

These MIDI files contain a 'call and response'.  A melody is played which can be repeated by the class.  If you load these sequences into a sequencer, you then have the flexibility to erase this if you wish, alter the tempo, change voicing etc.