We've got one DS 15 entering his Soph. year in HS next year. While we've had one DS 19 go through the same HS already, our youngest is highly involved in all facets of music (Band, Jazz Band, Choir, Marching Band, Show Choir and many of the corresponding honors musical events)
What we've learned in our school's system about such a child is that all those music activities are credit course classes, which leaves the precious class time slots open, all of which are filled with core classes. In order to make the schedule work, summer school Physical Education (required each year in our school district) is on the agenda just to fit everything in. The summer school PE class is at the same time as the local bank camp / jazz band camp, so we may being going out of the area or perhaps state to find another opportunity for those.
Our eldest DS 19 will be taking Calc 2 at the community college so he can take Physics 3 for Engineers the following fall semester at the state college. Again, scheduling problems often make our family look for alternative solutions. If you a parent of a GT child or children, expect the juggling and going to extra lengths to create opportunities and for appropriate education to continue well into your child's teens.