I am not sure I understand your situation. I am sorry you lost someone you admired and felt close to. But why have you shifted EC responsibilities to peers? Why are you unable to do your school work? Are you feeling paralyzed by the loss? ( I lost my Dad in high school so it isn’t like I don’t understand grief. )
Do you have to travel to the memorial service? Does the memorial service impact time available to do your work in a way that makes assignments impossible? Or are you emotionally unable to work? Have you talked to a counselor at school? That might help.
You seem to have had an unusually close relationship with your great aunt, and that may be hard to understand for some people, because it is not immediate family. That said, unfortunately, this kind of thing is not often accommodated. Believe me, I know. In fact, bereavement leave for those who are working seems woefully inadequate, much of the time.
Do you go to an especially rigorous school? I do think it would be fair to get an extra couple of days considering the Memorial Service.
Can one weekend have that great an impact on your GPA? It sounds to me like you need to talk to the high school counselor or if there isn’t one, maybe find someone in the community. Therapists can be helpful when grief affects functioning. A local hospice may have resources.