Can't speak for Sweden, but here in Australia, we went to our son's school and explained that we intended to take him out for three months for an extended holiday. His teacher had no issue with it and based on that, the principal signed off easily enough. For a struggling kid, there might've been push back, but you have to ask to find out.

When I was in high school, my parents took us out of school for 3-4 months to travel Asia including months in China. I've always appreciated them having done this for us. I was studying Chinese at the time, so it was an incredible opportunity on that front alone.