Hello! Yes, Royal Conservatory of Music certificates for grade 6, 7 and 8 count for high school credits (5 per year) of grades 10, 11, and 12 respectively.
The guidance counsellor at your child’s school would be your point of contact. They would have to assign the credits. Remember, for grade 6 & up, there are two exams: practical (usually can be completed as late as the first three weeks of June per school year) and the theory exam (usually held second weekend of May).
SO don’t wait until later in the year to cram in the theory! At this level it’s pretty difficult, quite abstract and academic, so you need solid familiarity with a process before you take a test.
Here is a link to Alberta Education’s Guide to Education. Information for this starts on page 70.