The relationship with Heather was contrived
I don't think I am being in any way controversial if I state that overweight social outcast males are not usually popular with the female sex. A boy in high school like Terri would seriously struggle to get anywhere socially with any girl, even if the girl were herself at the bottom of the social pyramid, and regardless of any good qualities that the boy himself possessed.
I therefore couldn't take seriously the idea that a very attractive blonde like Heather would be in any way interested in Terri, even if he did go out of his way to help her on multiple occasions, and even if her own behaviour had isolated her from her peers. It just wouldn't happen.
The relationship with Heather helped push the story along, but it wasn't believable and it marred the last third of the film.