First- my memory isn't all that good- I tend to check posters' records before replying.
Backstory usually isn't needed by the reader, to follow a story, in as much detail as it (the backstory) may be needed by the writer to enable him to write it. It's often boring when it appears before we have had a chance to connect with the character.
I don't want to know the backstory of someone who taps my shoulder in the street. I'd rather share an experience with him first, so I then might
want to know his backstory. And even better if I (or another character) has to tease it out of him or out of another character who is reluctant to reveal it, when it is relevant to the unfolding tale.
The bullying scene surely isn't backstory as such if the story chronologically follows on from it.