Prep School

Prep School is a miscellaneous quest in Fallout 4.

Detailed walkthrough
In order to begin this quest (note: in Pipboy it's named Kill Zeller), it is necessary to first complete the miscellaneous quest Traffic Jam. When turning in the quest, Deb will provide the trigger for speaking to Kessler.

Kessler is looking for someone to discreetly remove Bunker Hill's raider problem. In the past, they've paid off Zeller and his gang to leave their caravans - and the town itself - alone. Recently, though, Zeller's been demanding more money and then attacking caravans regardless. Kessler would like the problem to go away.

Zeller and his gang are located in the East Boston Preparatory School due east of Bunker Hill (across the water and just north of Boston Airport). The school has three floors and can contain anywhere from a dozen to 30 raiders. On the third floor, "Judge" Zeller has a makeshift throne and numerous prison cells in which you'll find the caravan prisoners. It's recommended that you clear the building of all raiders before freeing the prisoners, since they will run through the building and to the back entrance.

Bugs
If you previously cleared the location and decided to free the prisoners before accepting Kessler's quest, the miscellaneous quest item "rescue any caravan survivors" will still be on display after finishing the quest.