- For the episode of the same name see 15th Graders (episode).
The 15th Graders are a trio of students who have been held back for three years. As a result, they are nearly in their twenties instead of their mid-teens like most students at A Nigma High.
They have declared the hallway leading to the metal shop their territory, leaving every locker in the hallway visibly damaged as a result of them constantly punching them. Their rule of the school through fear came to an end one faithful day when Lee Ping walked into their territory while chasing a lead. They promptly challenged Lee to a fight and went on a water balloon rampage.
Though Lee attempted to avoid the fight by getting them detention, it only served to strengthen their dislike of him, as they were ultimately the victim of the pidgeon prank Lee pulled on them.
When the showdown finally came, Lee and Tina Kwee were able to whip the crowd into a frenzy, reminding them of every beating, wedgie and water balloon they had suffered at their hands. Though The Red Tazelwurm ultimately broke up the inevitable confrontation, their once mind-numbing fearsome reputation has been irreversibly damaged now that the people of A Nigma High know how to stand up to them.
- According to a Q&A session with Detentionaire co-creator Charles J/Charles Johnston, the 15th Graders live in a shared loft apartment in an old factory. Also, teacher consensus towards the 15th Graders seems to be divided, with some teachers feeling sorry for them and others being slightly afraid of them.
- According to a separate interview with co-creator Ranxerox76/Daniel Bryan Franklin, the 15th Graders are 19-years-old and have been held back for 3 years. This implies that they are repeating 10th or 11th grade for the third time.