Unless your gym teacher did anything or was trying to touch girls (or boys) or look down their gym shirts etc, i wouldn't worry about it too much. Men get easily aroused for no reason, often when they're not even thinking or looking about anything in particular. You say he just had a baby? He could have been thinking about his wife or anything. Don't take it to heart. Plus medically - men often become hard due to differences in temperature or pressure on the scrotum and shaft of the penis, and sometimes due to an imbalance in the chemical process from his brain to his genitals. If his wife was anything like me when she was pregnant, she abstained from sex all through the pregnancy, so his hormones could be going haywire a little due to pressure. i wouldn't worry about it. As far as i can tell, it was completely normal.