Article History
Received: 23 May 2019
Accepted: 2 July 2019
First Online: 15 July 2019
Compliance with Ethical Standards
:
: The authors declare that they have no conflict of interest.
: This present study was conducted in compliance with the specific requirements of Spain for retrospective studies. All procedures performed in this study were in accordance with the ethical standards of the institutional and national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.
: Informed consent was obtained from all individual participants included in the study, their parents and principals of all schools.