Here is the full list of all 26 Magical Diary: Horse Hall achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
You became a full member of the Rose And Wasp.
You helped Ellen form a school conspiracy
You broke your oath and lost your magic - for now.
You maxed out your Merits.
After a year locked in the school dungeons. Nasty.
You maxed out your strength.
You maxed out your smarts.
You received five detentions in one game.
You refused to attend any classes!
You maxed out your Weird.
You maxed out your Cute.
You retrieved the elusive stuffed bunny.
You maxed out your White Magic.
Oops! Accidentally married.
You maxed out your Green Magic.
You won the election for class treasurer.
Too many demerits ended your school year.
You maxed out your Black Magic.
You made it through the school year!
You had an escort for the May Day Ball.
You can't go home again.
You maxed out your Blue Magic.
You maxed out your Red Magic.
You maxed out your Stress.
Someone has locked lips with you.
You received detention