A person is guilty of criminal trespass in the third degree when he knowingly enters or remains unlawfully in a building or upon real property (a) which is fenced or otherwise enclosed in a manner designed to exclude intruders; or (b) where the building is utilized as an elementary or secondary school or a children's overnight camp as defined in section one thousand three hundred ninety-two of the public health law or a summer day camp as defined in section one thousand three hundred ninety-two of