TWO PUPILS from Monifieth High School last night won the grand final of The Courier/Royal Bank of Scotland Schools Junior Debating Competition.  Second year pupil Jonathan McGinty and his third year colleague Sarah Coughtrie talked their way to an all-expenses paid trip to Florida.  Six teams debated in the splendid setting of Lower Parliament Hall at St Andrews University, the highly topical motion being that, This house would make devolution a stepping stone to independence.  It was the first time that a team from Monifieth High School has won the competition. A team from Dollar Academy was runner-up.  After the results had been announced, the delighted youngsters whose huge grins gave away their emotions admitted they were ''ecstatic'' at the victory.   Jonathan is a keen war-gamer, basketball player and kung-fu enthusiast whose favourite subjects at school are computing and English.  Sarah, meanwhile, is decidedly musical.  She plays the lead trumpet in her school orchestra, sings in the choir and acts in school shows.  She enjoys Spanish, French and English.