The majority of 15-year-old students in almost all European countries speak the language of the PISA test at home …

„The PISA 2015 data reveals that the majority of 15-year-old students in almost all European countries speak the language of the PISA test at home, which is further considered as a proxy for speaking the language of schooling. The data reveals that, at EU level, 91.0 % of 15-year-old students speak the language of schooling at home.“
EU-Kommission (Hrsg.), „Key Data on Teaching Languages at School in Europe 2017“ (2017), S. 22
In Österreich sind es nur 81,2 %.