The thing about rumors is that they may either be based on truth, or they may not be based on truth at all and just assumptions.
While Morgan may have had friends who did heroine, it doesn't mean that she herself did, or maybe she did do drugs, but she didn't do heroine.
If Morgan did have a drug history, is Charlottesville known to the surrounding areas to be a place to go to get drugs?
When i was younger, i had an eating disorder and people spreaded rumors that i was on heroine, which i have never touched in my life. Rumors fly around by those wishing to make conversation and distract attention away from theirselves.
While we don't know for sure if Morgan did drugs and what the drugs were if she did do them, if she did ever have an eating disorder or was more thin than the normal weight for girls, i can easily see haters starting rumors about her that she was on heroine. Maybe it's based out of jealousy, i don't know the reasoning..
From what we've seen of Morgan's bedroom she has an eclectic taste in music which varies from Marilyn Manson to the Grateful Dead. I would say she took interest in being unique (her prom photo, she the only one of the 9 wearing a red dress, the rest wearing the same color) Morgan prided herself in not being the same as everyone else was. This isn't a bad thing, it shows she was in touch with herself enough to not be afraid to be different. Not everyone is like this, and some of those that aren't may have started rumors about her. Maybe they were jealous that they couldn't be like her as to even slightly break the rules of soceity's norms whatever they may be for them. You have a group of 9 girls, judging from the prom picture you have followers and then you have leaders. Morgan seems a leader in her own right.
This is all IMO...