So my overall GPA is 98%, but there's one grade of 78 on my transcript of my music class for just one marking period, the marking period follow the 78 for the same class is a 100. So can I still get into a selective college?
Yes. You may get asked about the 78 in an interview, because it's a deviation from your otherwise high GPA, but a 78 in a non-core class one marking period will not prevent you from getting into a selective school, even if you're applying to a conservatory.