So I am trying to write a story with some character who are going to school and I need some characters (from the show) around the CMC's age to function as bullies in the story.

The requirements are that they must be fillies but also can't be either Diamond Tiara or Silver Spoon.

6879419
I’d say, check the MLP wiki and look at the fillies known in the show. Just choose some that you find suitable, and, if need be, create them a fitting background.

6879423 Admittedly my phone is bad Showing wikis sometimes but I'll see. *Finds lots of Apple family relatives.* Hmm...

6879440
Hmm, a whole barrel of bad Apples? That ought to be interesting!

6879419

6879476 Indeed.

6879547 the image is broken for me, so I don't know.

6879476
Babs Seed is Apple Bloom's cousin from Manehattan and was once a bully because she was always picked on by the kids in her school and by Diamond Tiara and Silver Spoon in One Bad Apple. But Babs learned to stand up for her cousin and friends and is nice now and promised to tell her big sis if she's being bullied.

6879547
Cozy Glow is more of a manipulator of sorts. She will pretend to be your friend if you do something in return for her.

6879614
Yes, I know?