The twins are very young to be sent abroad...
Could the school be Institut Le Rosey ? A very posh and expensive school. Marie Chantal of Greece and her sisters all studied there.https://www.rosey.ch/
Exactly one of the schools I was thinking of!
The 2nd school is the one that Chris O'Neil attended.
The Institut auf dem Rosenbergís boarding students range from age 6 to 18.
 The Institut auf dem Rosenberg has a staff to student ratio of 1:4 and average class sizes of 8 students per class. 99% of Rosenberg students secure a university placement after graduation. Some 90% of students at the Institut speak 2 languages or more and 60% of the students speak three or more languages. The Institute is one of very few schools offering its students five different paths of education: