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