No mate i dont think he played for Under 19. He did well in a regional school tournament, was spotted. Worked in a short camp with Andy Roberts. Mashrafee claimed the camp changed his life. Was quickly drafted in the A team. Had a terrific tour in India where he troubled batsmen with his pace and bounce. He was so damn good he was fast tracked to the NT for a series against Zimbabwe. Zimbabwe were better than us at that point, and they got quite a surprise facing Mashrafee. Mashrafee was the only BD bowler who could make a batsman play on the backfoot and at will. In that series his fastest delivery was 147 kph, which got a huge roar from the crowd.
Imo Mashrafee is a special once in a decade talent. You just cant get a player like him just like that. But if you take his word, then the coaching by Andy Roberts helped him immensely.