Here is the full list of all 24 Kindergarten 2 achievements. It takes around 8-10 hours to unlock all of the achievements on Windows.
Get through your first day of school.
Help the Janitor win the war against Bob.
Collect your first Monstermon Card.
Die for the first time.
Collect the specimens for the science teacher and use them appropriately.
Help Ted prove his worth to Felix.
Help Buggs get his knife back from Penny.
Help Nugget dig a new Nugget Cave.
Help Felix get his full inheritance.
Help Ted foil his brother's plan.
Help Carla get the device that Jerome has.
Help Ms. Applegate get her fix.
Help Lily and Billy save the captured kids.
Change your outfit.
Collect 10 Monstermon Cards and make a full deck.
Win a Monstermon battle.
Win all the Monstermon battles.
Collect all 50 Monstermon Cards.
Collect all of the outfits.
Everything.
I'm so proud of you.
Found homes for all of Agnes's cats.
Saw something gross.
Remembered that Nugget likes flowers.