Big favorite of mine, as well. So here are some. Honestly, mostly friends rather than bullies.
https://www.asstr.org/~Kristen/21/mother.txt

It's interspersed throughout this writer's work.
https://www.literotica.com/stories/memberpage.php?uid=4668&page=submissions
Return to the Womb has a nice one. 2 Moms, 2 Sons, 1 Hot Night starts with a pair of cousins being 'educated' by their sexy moms. The protagonist's cousin, however, is more mature and has a bigger dick, and she's a size queen.

This one starts with that idea, heavy. It's a long, long story and I haven't read it all.
https://www.literotica.com/s/cougar-house-ch-01-milf-money

Purplemonkeydishwash does this a lot, as well. https://www.literotica.com/stories/memberpage.php?uid=1029823&page=submissions
Primarily in
https://www.literotica.com/s/betrayed-pt-01
https://www.literotica.com/s/feeding-mom
https://www.literotica.com/s/party-mom-ch-01 - Just this chapter, the rest gets a little weird
https://www.literotica.com/s/the-basketball-team-ch-01 - This has a girl who acts as the distraction

Literotica's thehumpman does this a bit too
https://www.literotica.com/s/katies-craving - long series but most of it fits
https://www.literotica.com/s/reporter-mom-ch-01

Last but not least, a girlfriend one
https://web.archive.org/web/20190421082905/https://gustavjorgensonauthor.wordpress.com/2019/04/14/girlfriend-sat-bullys-lap/

Love True Performance, and Obliviouscuckold is great too, wish he fully jumped on board the bully train.

The page was created so we can talk about a shared interest together. There's no demons, and no acting out, it's just fiction representing some of humanity's most natural interests: sexual arousal.

If you loved us you probably wouldn't be trying to convince us we're unhealthy and unhappy and scared. Proceed with no nut November if you want, but you can leave the holier than thou attitude anywhere else online.

I've been loving your stories, I wrote a comment on one of them (This one, in fact) on Hentai Foundry. Hope you keep writing 

Really well done. I normally like it more when the mom is more engaged but the ending was great.

This guy is an author who has written several bully fucks girlfriend type stories then stopped for a while. I noticed he finished a series recently and thought he might be getting back into the swing of things so checked his twitter.
https://twitter.com/londontwistsmut

It is right up this forum's alley, I'd be surprised if he wasn't reading some of the same stories and looking at the same captions we are. Hope he knows there's a market for this. If not, he may write on commission.

In complete agreement, moms at proms really interests me and those pics, wow.

Technically not a mom story. It's about a bullied teen and I guess she would be his sister-in-law. Her husband, his brother, commit suicide and they have fallen into a depression. In addition to that, he is being bullied by kids at school (kids surprisingly younger than him). She goes over to talk to them and gets won over and pulled out of her funk and into, well, a different kind of funk. It contains some weirdly young stuff, most of the site is Japanese (where that's common in fiction) though they're adding a ton of English content.

It's a surprisingly good story. There's definitely things I would change (If we were swapping her out for a mother, would it be better if the father died or a sibling?).

Anyways here it is, on Pixiv: https://www.pixiv.net/novel/show.php?id=12172813

After finding that gem I looked around and found a couple others more close to our interest. They aren't as good but the writers were trying.

Mother Married to Bully part 1 though it looks like they've given up on any part 2:
https://www.pixiv.net/novel/show.php?id=9915798

And Mommy's Humiliation, which seemed like it was going another route by having the bully be not only a cousin, but a girl.
https://www.pixiv.net/novel/show.php?id=11343219
https://www.pixiv.net/novel/show.php?id=11352782
Spoiler and Warning: It doesn't get to sex. It's possible this is going to go an entirely nonsexual way but so far it's up in the air. The artist has some (3d) images in his gallery that go along with scenes in the stories.

Very hot story.