Well, I don't read a whole lot of romance comics, but of the ones I do, there are a couple who are set outside of school-settings!
Blossom Boys has one character who is in college, but only a small percentage of the scenes are actually in a school environment; they're mostly set in a florist's shop, or in other non-school locations.
Erie Waters is a paranormal romance set next to a lake, and has NO school-elements at all. The main characters are strangers to each other at the start. It's finished, and lovely, and you should read it.
Dragon Husbands is set in a scifi city where real plants are so rare they're almost mythical - also there are dragons. Absolutely no school-elements at all, and the main characters are (kind of?) strangers to one another at the start. I will say that there are some more mature elements to the story, so proceed with caution if you're not over 18.
Hades' Holiday features characters (based on Greek myth, but not sticking 100% to their mythological origins) that already know each other, but has no school-elements at all.
I think the reason that school-settings are so common is that a.) 99% of the readers will be familiar with it, as most of us have at some point been to school. It's a setting we're so familiar with that the creator doesn't have to explain it. And b.) it gives the characters an external reason to meet: they might be in the same class, they might be student and teacher, they might be fellow teachers, they might be a teacher and a parent, etc., etc. And they have to meet because the school structure requires them to be in proximity to one another.
It's an easy way for the creator to set up the meeting of the characters.