Rather than see the endings as "good", I think it might be better to refer to them as a "fresh start" instead. However, the experiences Sam endured at the school will probably haunt her forever, permanently affecting her behaviour in the future. In one ending, she is more likely to never trust another person again because of her friend's betrayal. In the other, she may cut herself off entirely from any reminder of the school, including from the people who were genuinly trying to help. Based on the details provided, it seems she already had. Either way, I doubt she will return to being a bully, but I also doubt she will properly move on from being a victim. She knows how awful people can be - how awful she herself has been, and will thus have an even harder time remembering that there is good in people as well.
On the other hand, I suppose I can understand the "good" impression people have found. Were Sam to remain at that school regardless, and the bullying problem had been "handled", she would still be attending a school in full knowedge of being hated with only one support (a previous victim, in fact) to rely upon. She'd never be receiving the opportunity to move on and become a truly better person. We don't know the reason why Sam became a bully. From Molly's perspective, it seems Sam hadn't always been that way. Molly believes that Sam can become better and has the capacity to change for a reason. Perhaps by distancing herself from the physical manifestation of her problems, Sam can finally begin to view her choices and situation more objectively in a means to eventually handle her burdens properly. Perhaps after some time away, she'll be able to change for the better and actually find some good friends to surround herself with.
To be honest, I'd really love to see how Sam's future turns out. I'd like for Molly and Sam to accidentally reunite and discuss this moment in time fully. Why Sam was mean to Molly, what she learned from the experience, whether she successfully managed to move on or not, how Sam views Molly now, and if the two can become proper friends now that the past is behind them are all questions that I'd be interested in learning the answer to. Of course, I wouldn't want them to reunite until after they've become adults, since I feel that anything sooner may just be too soon. On the other hand, once Molly has matured some, she may be able to provide Sam the proper help that she needs if Sam is still caught up in the past. As seemingly the only person who truly treated Sam with respect during that time, Molly might be the only person Sam feels capable of relying on to help her move on from that time.
In a sense, I guess that I'm saying more could be done to the story, speaking from a story's standpoint. However, it could dimish from the message you are clearly trying to convey, but I still think a sequel might be appropriate in this case depending upon how you go about it. You've shown us the power of words when brandished to hurt, but have yet to show us the potential of words when designed to heal and to help. Sure, we saw some of the possibilities between Molly and Sam's interactions throughout the story, but we've yet to see how a single person can and does make a big difference. I think that pusuing Sam's story following this event, told from her perspective, could communicate this message strongly. Your point is that everything spoken has the strength to greatly affect another human's life. By exploring the after-effects on Sam's life, you could simply be proving the other side of your argument. Yes, humans have a nature of focussing more on the negative, but we cannot forget the impact of the positive. One comment from a complete stranger has the potential to make a person's entire day. I think it's important for everyone to know that and think that you are in a great position to spread that knowledge.
(And you thought your comment was long. Hope I don't intimidate you with this and that you realize these are merely my musings/suggestions and not demands. Just in case you weren't quite as finished with the story as you initially thought. At the very least, it can help you pass the time if you need it. Thank you for listening and replying to my first review. I hope you have yourself a wonderful day between our next interaction.)