How to unlock: Talk to Kessler in Bunker Hill (M5,25), after you complete the quest for Deb (clearing the location of ghouls)
It is recommended that you complete this quest before the Battle of Bunker Hill.
First, talk to Deb and complete the quest that you received from her. Only after you do that, Kessler gives you hers. With your Charisma high, you can additionally receive 2x Med-X, 2x Stimpak and some 5mm ammo upon accepting the quest.
Go to East Boston Preparatory School to the East. Inside, you encounter a dozen-or-so gunners at each of the floors. You need to make it to the top, where you find Zeller. He is not much stronger than the rest of the crew.
After you have eliminate all of the enemies, free the three prisoners. You can do this by lockpicking the doors of the holding cells (Easy), or with the keyring that you pick up from the desk with the terminal. Eventually, leave the building and return to Kessler.
Rewards for completing the quest: Experience points, 400 caps, option to set up Trade Caravan Posts in settlements- thanks to this, caravans can come to towns, where you can trade with them.