A letter went out to Pulaski County School District teachers today informing them of the end of contracts negotiated by the Pulaski Association of Classroom Teachers, effective June 30, and the beginning of contracts shaped by a personnel committee. The letter upset some teachers who read it, according to one note received here, as a termination letter. It's not, says a district spokesman. But there are unhappy labor relations in the district thanks to a narrow board majority's decision to oust the union. The letter is a formal notice of that change and an attempt to make the changeover legal in the eyes of the court where the union has challenged the action.