Let's say I am in school, and my parents want to visit my school to find out how I am doing there. Which one of the following is the most natural way to communicate the idea?
My parent are going to school.
My parents are going to the school.
I think the second one is the way to go as my parents don't study in school, so the use of the definite article is necessary.
You are correct that (without other context) “to the school” would be the preferred alternative to convey the meaning you described.
Your own description shows an even clearer way of communicating your intended meaning:
My parents are going to visit my school.
I should add too that “my parents are going to school” won’t always mean that they are attending school as students themselves. It depends on context. For example this is possible:
My friend asked me, when she saw my parents leaving the house, “Where are they going?”. I replied, “My parents are going to school. They want to talk to my teacher.”
But “going to my school” would be more likely even in this context.
