Ren received the other merits that weren't related to killing the alien forces. They came from things his actions had caused, like securing an entire ship for the research department and so on. These merits were shared amongst all the soldiers that contributed in the battle, including the other students who fought, but it was only natural that Ren took the largest chunk of it.
With that alone, it was enough for him to rise up to 11-star Private. With such rank, he was well above his mates and most of his seniors and eve had the right to other them… lightly. Even though they were his seniors in school, they were still his juniors in rank. Since 11-star Private was something that was reached only when one was in the final year at the academy, it was really something for a first year to have.