The people that do the selections for ROTP understand that applicants are young, and were very recently young teenagers who may have gone through an "I don't care" stage, and their marks reflect it. They will not turn you down because you had a bad semester. They will love to see that you are improving your marks. However, you also have to consider what you are up against. If there are students who did not have that stage, and had great marks all through high school, they will obviously be more inclined to take that student.
You are doing the right thing though. Even if you don't get in, you are improving yourself, which is great. That means you will have a good grade 12 year, which means if you don't get in your first try, you can try again, and this time they will recognize that you did turn things around and now you are committed to good schooling. Trying to improve yourself in anyway is not done in vain. It makes you a better person in general, and it only strengthens your application. You would be in a lot more of a pickle if you didn't turn your marks around at all.
My tip to people (especially to people who want to be pilots) is that you shouldn't worry about things you can't change. You should be worried about what you can change. If you think you have poor eyesight, you can't change that. If you had a bad term in the past, you can't change that. But what you can do to make yourself as competitive as possible is do better in school NOW and in the future, and start volunteering or taking leadership positions, or playing more team sports.
In other words, do everything you can to better yourself in all aspects of life, no matter how small, because everything counts, and everything you give them will be one more reason for them to overlook your bad marks in earlier high school.