The best way to safely extract an educated child is through the Onderwijsbegeleidingsdienst, or Educational Advisory Servies. They can test the educational level of the child. Beware, some of these workers will try and provide tests, such as an IQ test through the medium of Dutch. If a child does not read or understand Dutch they will be given a low mark, which can affect their access to education. No account is made for the lack of Dutch language, there are no alternative language tests available (at least in 2004).

However, if you can ge educational references from a child’s previous school attesting to their educational level of achievement, it is possible to ask the OBD for assistance to gain access to a Montessori school These schools have been successful in providing concentrated Dutch language education to children and then enable the child to spend less time on their native European language.

The OBD helped me take my two girls out of the ISK into Montessori to do a MAVO. We then returned to the UK where they studied further and have both graduated from University.
Bear in mnd that the ISK insisted that the two children had a low IQ and were destined to LBO, lower level education.