For Valencianos their first language is Valenciano. The majority don’t learn Castellano until they go to school.
I do not believe that the school system is biased against foreign students, the system is what it is and maybe people should consider that before they come to live here in Valencia.
There is a large part of Spain where the first language is Castellano.
I belong to a Spanish reading group and they speak in Castellano solo for the benefit of another English woman and myself. Otherwise, I know that they would speak Valenciano in fact they do lapse from time to time.