The Colegio de Bachilleres del Estado de Guerrero in Mexico, which offers upper secondary education, has an online verification service for recently-issued Certificado de Estudios:
Enter the student's CURP number from the document for verification.
The site does not confirm the subjects and grades, but it does confirm the Folio No. of the document, as well as whether or not studies were complete, and the date of completion.
The same site can also be reached from the main institution site, under Consulta Certificado.