Can 15 years old student study high school aboard ?

Aboard what? Boat? Plane?

Most places, no. You can only obtain a student visa for foreign exchange student programs or boarding school. Sis must be your legal guardian to register you for school (& pay non-resident tuition) or to obtain health care for you. And you must get private health insurance coverage, or sis must pay cash if you became ill or injured.

You can do a study aboard program
You can attned a private boarding school

Yes, 15 year olds can study abroad as long as they have enough money and the program they're enrolled with is for high school students and up.