At my school, improvisation is stressed from day one of freshman year. Luckily both of my directors were professional jazz guys around town (trombone and percussion) before coming to teach. We have an improvisation class once a week for beginning students, and many different combos that help us develop. We spend a bit of time in class but almost the whole band is taking private lessons that help with the improvisation. Even still, all of the solos go to me, the tenor 1 player or the trumpet 2 player.