Here is the full list of all 11 Really Another True achievements.
Open the entry door.
Complete the game.
End the game with a normal ending or higher.
Complete the game on hard.
Pour the poison into the kitchen sink.
Kill the boss on the first floor.
Put a chair for the hanged man in the alternate staircase.
Return the wizard marker to the school locker.
Find the hammer.
Get 10 stars in statistics.
Eat cake in the kitchen.