B.B.
My 3.5 y/o son has been in ST for about a year now. He goes twice a week to an elementary school, and he has 1 other girl in his class. It's only 25 minutes each session, which is about the max of his attention span. He loves it! To him, it is definitely more play than work, but he has made a HUGE improvement!!
Something I do is to never correct him if he says something wrong, but to repeat it in the correct way (hope that makes sense). I don't tell him he's wrong. And with my son, he leaves off the ending and some beginning sounds, so I try to exagerrate certain sounds when talking to him.