Midwich Elementary School is the new map that takes place inside a giant school with an open courtyard in the middle.
It has two main floors. Then it has the basement and one staircase with a dead end that would lead to a third level.
The two rooms of interest for this achievement are located on the second floor.
The "music room" has a piano in it.
The "chem lab" is the one that has orange/red trays on the tables and you should notice a periodical table hanging up on one of the walls, plus a hole wedged between two tables that you can use to fall to the first floor.
Sometimes a gen won't spawn in one of these rooms, I don't know if it's possible that no gen will spawn in either. If you use the stage offering and get the map then one will appear in both every time.
A green rarity offering exists for the map called "Mary's Letter".
You can do either one of those two gens, you don't need to get both.
Escape via hatch should still allow you to unlock this but its been glitchy in the past.
If you get stuck as the last survivor alive and you still need to power up one of the gates, you're basically screwed as the gates are in very close proximity to each other.
You do not need to own the DLC on the store or need to buy the new characters in-game to unlock this.