I think it's worth just going to your teacher and asking them what you can do to improve your grade. Some teachers won't change your grade from what you got at mocks as your mocks were actually in exam conditions, but some don't mind changing it based on your performance in classes. Going from a D to an A might be pushing it though, perhaps a B is more likely, but, again, it's better to directly ask your teacher, or maybe the member of staff that sends off the applications.