My answer 'This is a boys school' was rejected.
In England we say 'boys school' or 'girls school' to describe a school where only boys or only girls attend.
Does the Portuguese sentence have a different meaning?
In the USA, we also say boys' school if it is a school for boys and girls' school if it is a school for girls. I think it would be alright to translate this sentence as "This is a boys' school" or "This is a school for boys"