The Secretaría de Educación of the state of Zacatecas in Mexico offers online verification of educational documents for basic, secondary, and higher education, including Preescolar, Primaria, Secundaria, Bachillerato, and Normales (teacher training). It's not clear how far back this service goes.
The link for verification is http://certificacion.seduzac.gob.mx/. Enter the student's Clave Única del Registro de Población (CURP) listed on the document, first and second family names (Primer and Segundo Apellido), given name(s) (Nombre[s]), and year of issue (Año expedición). Then choose the level of study and click Consultar.
The result brings up a copy of the educational document, which can be downloaded.
Recently-issued documents may also have a QR code which can be used for verification, but check carefully to make sure the code directs you to an official site.