It is generally held that schools should be politically neutral. Here is
a private Catalan nursery school (Les Alzines in Girona) with banners
saying 'Towards Independence' in Catalan and English. They are in a
prominent position beside maps and other Catalan material. The school is
connected with the extreme Catholic group Opus Dei.
Here it is again in a tweet from a fortnight ago.
Here is a classroom in a Barcelona secondary school (IES La Sedeta) showing an 'estelada' (independence flag) in the top right (YouTube)