BOSTON — From his first days at Harvard, Merrick Garland’s classmates pegged him as a star. Smartest guy in the room, self-confident, easygoing and thoughtful are some of the compliments they still pay him 40 years later. For Garland, President Barack Obama’s choice to fill a vacancy on the U.S. Supreme Court, the bonds formed at Harvard College and Harvard Law School have stayed with him and come into play during a career in private practice, as a prosecutor and on the bench. Classmates said Garland followed a straight and narrow path, even as a young man during a tumultuous time marked by Watergate and protests over the Vietnam War.