If someone is in the same grade as me, but they're older than me do I use 존댓말 or 반말?

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That varies from person to person.

We often have a dilemma like this.
Person A is 22, and a 2nd year in college (military service)
Person B is also 22, and went to the same high school as A, but a 4th year in college cuz no military service.
Person C is 20, and also a 2nd year in college

A and B talk to each other casually, because they're the same age, and they went to the same school.
Meanwhile, C doesn't know what to call A, because he's in the same grade as them, but older than them.
If C talks to A casually, then B is going to think "why are you talking casually to my friend?"
But if C talks politely to A, then his friends, some of whom may be 3rd years in college, will have to also talk politely to someone in the lower grade.

People often resort to talking in formal voice in front of people, but then using casual polite when they are alone, or together with people in the same grade