Looking for an American high school 90’s movie

Hi everyone,
I’m looking for a movie that must be from either the 80’s or 90’s, mainly set in high school, a small town in the U.S. There’s one girl, black hair, well to do, and a boy up to no good with brown hair. They don’t get along but are partnered together, science I think. They become friends, sort of but life happens and they lose touch.
The boy reappears as a Mormon or something near the end, and then she hears he died or disappeared… I can’t remember.
The last scene is of her after giving birth to her firstborn and naming him the boy’s name which is something like Thayer or Thaddeus…