Back to the nature of scales, the pentatonic and blues scales have inherent melodic elements built in, the skips and jumps built into the pentatonic and blues scales and the flat 5 in the blues scale. Just by playing these scales up and down they automatically suggest certain melodies. The major scale and its modes on the other hand are made up of successive small intervals, and when every note is played in succession they are less suggestive of melody by comparison. The best solution would be to learn the elements of melody, to learn to use larger intervals appropriately, add in chormatic neighbor tones, etc. However, for those students who need a leg up there are the hexatonic scales.
Some of my favorite settings for teaching improvisation are the 12 bar blues progression and dominant chord vamps. Either way you have to learn the mixolydian mode to completely understand your scale/note options in either of these settings. Following is one version of a hexatonic scale drawn from the C mixolydian mode:
This hexatonic could be used over a C7 (C dominant seven), and us built using scale degrees 1, 2, 3, 5, 6, 7, 1. The built in skip from degrees 3 to 5, which also avoids a tension note, gives the scale some inherent melodic content, and students can more easily begin using it in improvisation. Using similar methods hexatonics can be created for other scales and modes as needed.