This meaning of this poster, as far as I can tell, is two-fold. Firstly, it is a statement opposing religious education in general. Secondly, it is a statement in opposition to the recently introduced automatic enrollment program (TEOG) which places students into secondary schools. This controversial program has erroneously placed non-Muslim students into religious high schools. It has also placed students in schools located at great distances from their homes. This poster was found in the Okmeydanı district, a working-class area with a mix of religious and political groups, where leftist politics often finds support. English-language articles on this topic in Turkish online newspapers can be found here and here.
I came across this mystery in the Okmeydanı district, not far from here. No further comment.