The man was arrested on charges of "resistance" and "disobedience" after he tried to break up a fight between his four-year-old grandson and another child outside the school gates in Malaga, southern Spain.
A "scuffle" broke out between the two children outside the Colegio Público de Educación Infantil y Primaria (CEIP) on Tuesday lunchtime.
According to school sources quoted in Spanish media, the grandmother of one of the children confronted the parents of the other, "hitting them in the face" and pulling their hair".
Police were called to the school and arrived just as the woman's husband - the child's grandad - was trying to break up the fight. He reportedly resisted police officers when they attempted to pull him out of the scuffle.
The secretary of the school told Europa Press that the parents involved in the fight came to the school on Thursday and "apologised" because "it was not appropriate behaviour for outside a school".
The school board was meeting on Thursday to discuss whether to take measures against the parties involved, which could include banning one or all of the adults from entering the school.