You will be fine. I did HSC and CPL IREX at the same time. At the end of the day HSC means stuff all. Just get enough to get into uni (if thats what you want) nothing more is needed. I have never been asked what grades I got in High School ever.
Try and stick to a schedule like 3 lessons per month or more as it will be easy to learn if you are having consistent lessons and not having to relearn stuff if you have 3 months off... It is also a good way to break up the study at school.