Hi Stephan, and welcome to the forum. Each of our three boys attend the Bali Hati School which is just outside Ubud in Mas. We are very satisfied with the curriculum and the teachers. The tuition is also very reasonable. The mix of students is excellent, comprising of pure Indonesian, mixed Indo/Western, and pure Western.
Currently the school only runs through grade 6, although plans for a middle school are well in the works.
You can check this school out via their web site. When in Bali, if you would like to meet our boys and visit the school, let us know. Cheers!
http://www.balihati.org/education.htm