The Merrick is certainly, at 843m, the highest hill in southern Scotland, but to my recollection it can't be seen from Newton Stewart. The hill most clearly visible from the town is Cairnsmore of Fleet (711m).
The Douglas Ewart High School can claim at least one illustrious former pupil. James Mirrlees, a Nobel prizewinner (Economics) was educated there in the late 1940s/early 1950s.