Schools is tough on everyone.
Teachers don't like it because they never no how many students they will have because kids will go back and forth between the two sessions. The best way to provide two Sunday schools and three worship services is to have Sunday School at the early and late worship hours. This way, when you make the shift from one hour to two hours, everyone has to make the change. Otherwise, none of your existing adult classes will want to move to the second hour. If you start a second Sunday school, make sure you intend to start a class for all ages.