I forgot the title of this film I watched on tv in brazil several years ago. I remember it was old (felt like 80's or 70's) and it was about this boy and his teacher in the middle of a shape shifting alien invasion. in the ending his parents made him believe that it was all a dream but in the last minute he turn around and find out his parents have been replaced. the film ends with a frozen shot of the kid screaming but we don't know what he saw when he faced his parents.
Wild Guess. "Invaders from Mars". https://www.imdb.com/title/tt0091276/
In this remake of the classic 50s SF tale, a boy tries to stop an invasion of his town by aliens who take over the the minds of his parents, his least-liked schoolteacher and other townspeople. With the aid of the school nurse the boy enlists the aid of the U.S. Marines.
The thing about the parents made me think of that one.
Much as you remember, it ends on him rushing into his parents' bedroom and screaming without revealing what he sees.