"High school sweethearts" is a set expression in contemporary English.

Thank you, Zowee. Is there just one word with same meaning? Dictionary says lover is a relationship outside of marriage, but I am looking for a word to describe a normal girlfriend-boyfriend relationship. For example, they were high school sweethearts

Lovers?

You could say 'lovers' or 'in love'. Or perhaps 'they are together', or, 'they are a couple'. If you can give us a sentence it would be easier...