Tommy is presented as one of life's outsiders, a chronic misfit who doesn't really belong anywhere. By showing Tommy as a victim of merciless teasing and mockery, Ishiguro makes him a sympathetic character, despite his regular childish tantrums.
Although Tommy attended Hailsham along with the rest of the clones, there was always a sense that he was somewhat apart from everyone else. This would explain why he was always marginalized and singled out for bullying. For Ruth and the other clones, their whole identity is intimately bound up with their time at Hailsham. If Tommy was never a "real" Hailsham student—as Ruth seems to think—then the implication is that he lacks the kind of stable identity enjoyed by the others. Our sympathy for Tommy is heightened by the fact that he has to become a donor before the teasing stops. In other words, he has to go through the process of having vital organs removed before he's regarded by the others as truly one of them.