A boy gets ridiculed for dating a girl who is considered trash and EASY then he falls in love with her - local boys kill her - somehow boyfriend is in jail, breaks out, robs a bunch of flowers to put on her grave. Very sad...

Buster and Billie (1974)
Wikipedia synopsis:
The film is set in a small Georgia town in 1948. It follows the adventures of Buster Lane (Jan-Michael Vincent), a handsome, popular high-school senior, who is engaged to be married to his high-school sweetheart Margie Hooks (Pamela-Sue Martin). He is the 'big man on campus' and the leader of his group of friends. Buster, though, is different from these other young men.
Buster's friends, being sexually repressed, often visit a girl from an underprivileged background named Billie-Jo Truluck (Joan Goodfellow) who dourly gives them what they want. In the meanwhile, Buster becomes disenchanted with Margie's refusal to have sex with him, and begins seeing Billie in secret.
In the beginning, he sees Billie just for sex, but eventually finds himself falling in love with her. Buster is so taken with Billie that he decides to break off his engagement with Margie to be with Billie. Billie finds another lease on life with Buster, and they are happy for the first time in their lives. Happiness for them, though, is short-lived.
Buster's friends are extremely jealous that they cannot have Billie for their own use anymore and decide to teach Buster a lesson. They corner Billie one day when she is out, and, when she refuses to submit to them, they rape and kill her in the heat of the moment. Buster eventually finds her dead and is hysterical. He then goes to the pool hall where his friends are, with the guilt evident on the faces of the main perpetrators of the crime. Enraged, Buster kills two of them, while injuring the other two.
He is then put into jail for this but is released. He rips up entire garden beds in the town and takes these flowers to Billie's graveside in her memory.

Buster and Billie - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Buster_and_Billie)

I recall when this was a "dark horse" hit at the drive-ins.