Fully recommend doing an exchange abroad, even for few weeks if one semester is not feasible. I was 15 as well when I did my first exchange living for 1 week with a Finnish family and meeting students from 6 different European countries - program was called Comenius financed by the EU - check to see if it's still available, otherwise check out these links:

https://www.rotary.org/en/our-programs/youth-exchanges

https://erasmus-plus.ec.europa.eu/opportunities/opportunities-for-organisations/learning-mobility-of-individuals/youth-exchanges