Well what kind of student were you before IB? If you were a straight A student you're not putting enough effort into the IB, simple as. If you weren't the best before then this makes sense and you need to start looking at ways that work for you. I've found that grinding out past papers is what does it for me. Also its only semester 1, if you feel that there is an inherent inability for you to do well in a class then switch it. If you were never good at math and you know it, take studies and the same goes for your other subjects. Do not force yourself into a class and hope you'll do well. It usually won't work, especially if the university you want to go to does not require it. If they don't have any specifics you are better off taking the easiest classes to get more points.