At the age of 12, children go to one of the following types of secondary education:
International children aged 12 and above go to an international bridging class called Internationale Schakelklas (ISK). Students stay in the ISK for approximately one or two years, depending on their level, and the school will test them every three months. At the ISK, a lot of time is spent learning Dutch the first year, and there is less time for the content of all the other subjects. Once they finish the ISK, they can start at the appropriate secondary school level.
There are more and more bilingual secondary schools in the Netherlands. In bilingual secondary schools, at least 50% of the subjects are taught in English. The acquisition of a second language (English) is not the only goal in bilingual education. The schools also usually have a more international focus than regular Dutch schools.
The children speak English during, for instance, geography or history lessons or during physical education. On the other hand, the schools still follow the Dutch curriculum, so almost half of the subjects are in Dutch, and the children need to take the Dutch school-leaving exams.
You can find a bilingual school in an A-Z list of cities here.