"Free Leo Chindamo" appears in the opening credits of the N-Dubz star's new video 'I'm Coming (Tarzan Part 2)'.
Chindamo stabbed headteacher Philip Lawrence to death in 1995 outside the same west London school that Dappy attended.
Lawrence's widow Frances told The Sun: "I haven't heard the song yet. But I don't think it is right someone should be making money from using his name in a song."
Chindamo, who was 15 at the time and is now 31, served 14 years in prison for the murder and was released on licence in July 2010.
He was jailed last year after breaching the terms of his licence by meeting with a gang member, breaking his curfew and drinking on the streets.
MTV reports that Dappy apologised for his actions in now-deleted tweets: "Just wanted to say I honestly wasn't aware he did that, I knew his brother from when I was young.
"In no way did I intend to cause any upset and certainly didn't know about his past. My heart goes out to Mr Laurence's (sic) family and I know the pain of losing a loved one, sincerely Daps."
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Tulisa Contostavlos recently condemned the track.
Watch the music video for Dappy's 'Tarzan 2 (I'm Coming)' below (adult content):