mark west , 32 , was sentenced tuesday to four months jail and eight years probation for having sex once with a student , 18 , during 2012 prom event .
judge said west made ` a poor decision to have an inappropriate extramarital affair ' and that the girl was graduating just three weeks later .
the girl admitted to police she seduced west and met him in the office .
west is married with a son and resigned from spring high school in spring , texas , following his arrest .
staff reported other ` inappropriate ' behavior with female students .
west apologized , saying it was a ` selfish , impulsive decision '