Once you reach the final dungeon of the game (You might have to go all the way in, save, then get out), go back to the workshop, look up the tutorial crap and there will be a "???" entry. Look at it, then go where it says, the old school house closed area, to the end. There will be a portal.
Since you're on the 2nd playthrough, I think you can go there the second you're able to get into the closed area, so....