I like to see a story that doesn't start out as a "bullying" story but slowly turns into one.

A happily married couple with a son and a daughter.  The mother is at a school event of some sort for her son and ends up meeting and being charmed by a student in the school that's a year or two older than the son.  The son and this student don't have any sort of preexisting relationship.  They're aware of each other but not in the same circles.  Ultimately the mother runs into this student a few more times and ends up being seduced.

It turns into a bit of a cuckold story as she's fucking her new "boyfriend" behind her husband's back. 

Proceeds this way for a while and they start fucking at her house when no one else is home.  The "bully" convinces the mom that they should try to fuck in every room of the house.

They ultimately get to the sons room and the bully realizes he's turned on by dominating the son by fucking his mom right in his own bed.  He pulls out and cums all over the son's desk and belongings.  The mother is a bit annoyed by that but doesn't make a big deal of it.

This is the point where the bully hatches the plan to have the mother be his accomplice in "bullying" the son.  The wrinkle is that (at least at first), they're just doing mean things to him without him knowing who's responsible.

This could set up a whole series of "pranks" they play on him that would gradually ramp up in intensity and meanness.

Things like:

 - Bully fucks the mother with various items belong to the son. Basically anything that will fit in her pussy or ass ends up inside of her at some point.  Maybe the son wins a medal or some other kind of prize for winning a chess competition and the mother steals it and hides it up her ass all day while the son is freaking out looking for it.  There would be a scene where the mom is trying to "help" him find it but the medal is up her ass and the ribbon for it is just slightly visible above the waist of her pants

- The mother and the bully log on to the son's social media and cause some kind of havoc for him that way (note:  this is stolen from one of my favorite scenes in Conflicted)

- The mother sets some sort of "trap" or something that would cause the son to fall and maybe hurt himself a little.  Before setting the trap she hid a camera so they could record the fall.  They watch it on the TV later in the sons room while fucking on his bed.

- The mother installs some kind of spyware on the son's computer and they use it to control and record his webcam and ultimately get a video of him naked and jerking off to porn.  It might take a while, but the mother ultimately decides to upload it somewhere on the web so others can see.  She wrestles with the idea of uploading to a place where his classmates would be guaranteed to see.

- And so on....

I have no idea how it should end.  That's always the tricky part, I think.  The son would have to find out in some spectacular fashion I'd imagine,

Also not sure if I'd want to mother to ultimately corrupt the little sister for her help in bullying the son, or have her start doing mean things to the sister as well.