The 6th Graders were one-shot villains in Baka and Test: Summon the Beasts and the main antagonists in the flashback episode "Yuji, Shoko, and their Childhood Memories".
In the episode, they encountered Yuji and complained on how he had better scores for a 5th grader. Later, they decided to vandalize his stuff in the classroom and write with a permanant black marker saying "I like to cheat on my test". When Shoko attempted to stop them, she tripped and accidently wrote on one of their shirts. Yuji shockingly finds out, but had to make an ultimate decision first: To tell a teacher or to settle things himself. He chose to settle things himself, because otherwise, Shoko would've been transferred to an all-girl school. After forcing Shoko to go home (for safety reasons), Yuji got in the fight with the three bullies. It's unknown what happened to them afterwards, especially in the present.
This led Skoko into dedicating her life into marrying Yuji while becoming an abusive sociopath.