Jack is shown having to rush to make sure he caught the Titanic before it sailed. Many other steerage passengers board along side him. In fact, all steerage passengers were aboard well before the ship sailed; first class passengers did not begin boarding until all the steerage passengers were aboard.
First of all, Jack is the last person to board from that gangplank. Second, if they had showed the proper boarding order, the beginning of the movie would have been three hours in and of itself!