What is the ruling of the Shari'ah regarding a young man marrying a girl whom his parents reject, claiming that she is of a lower social standing than them?

Marriage may sometimes be obligatory, for one who longs for it, is capable of it, and fears falling into temptation, because safeguarding oneself from the forbidden is a duty. Since you are capable and fear temptation, hasten to marry to protect yourself. Your parents have the right to prevent you from marrying a specific woman, so try to convince them about the girl you want, and if they do not agree, then look for another.