I'm going to go conspiracy theorist here.
@SomervilleDan said they got the report cards legally. We know that the pests had been engaging in group texts with members of Fat's family.
It's possible that they also texted Fat's family individually. We know that they confronted Fat's family with his failings. I theorize that one of these conversations touched a nerve, not against the pests, but against Fat. I theorize that one of Fat's family members sent the report cards to the pests. I'm going to guess it's the brother because of Fat's later comment about him to Somerville Dan.
Knowing how Fat has to be better than everyone, even his Twitter friends, I can only imagine what it was like growing up with him as a brother. He must have been constantly belittling his sibling to salve his own ego. The brother likely gets great satisfaction watching Fat today blunder about from one obvious trap to the next. Watching his childhood stupidity get exposed is probably an act of healing.