Translation:El maestro es mexicano.
"La maestra es mexicana" is also a good translation.
In Spain, we say:
Méjico = Mexico
Mejicano = Mexican (man)
Mejicana = Mexican (Woman)
As you can notice, the X is substituted by the J
Mi maestro es mexicano. Why not?
Because the sentence is "the teacher...", not "my teacher..."
Maestro = man
Maestra = woman