These guides have helped me throughout my own playthrough so far, so go and give them a try!

• Monster Prom's Stupidest Pop Quiz Ever(tm): A Guide by Dayashi
  
• Stat Requirements for Prom by Coleman
  
• Secret Endings Guide by Crookedmook
  
• Notes on Dating by Azerith
  
• Monster Prom: Ultimate Events Guide by princess monster

These achievements come this time and patience. Just try your best!

List of Ending Achievements:

• That was unexpected: You unveiled the first secret ending.
  
• Ending explorer: You've already unveiled half the secret endings. Way to go!
  
• Ending connoisseur: Whoa, you've seen ALL THE SECRET ENDINGS.

List of Event Achievements:

• Whoa, you really like this game!: You've seen half of the events in the game. Get some rest.
  
• Go play outside!: You've seen ALL THE EVENTS in the game! Just wow.

List of Outcome Achievements:

• Honey... this isn't healthy: Okay, you've seen all the outcomes in the game. We didn't think we'd see this day!

Is self explanatory, just go to one of the locations below for all turns, quickest way to do this is by going into a multiplayer game and choosing all four of our main characters, Oz (Yellow), Amira (Red), Brian (Green), Vicky (Blue), and sending them to a location for each of their turns.

• Bookworm: You went to the LIBRARY all turns.
  
• Dodgeball God: You went to the GYM all turns.
  
• Monster Prom: The Musical: You went to the AUDITORIUM all turns.
  
• Nerd!: You went to CLASS all turns.
  
• Party 24/7: You went OUTDOORS all turns.
  
• Small bladder: You went to the BATHROOMS all turns.

At this time, you can also commit one character to be the one to speak to all of the minor characters. This can be unlocked in the original game, and doesn't need the DLC to be completed.

• Outcast: You always chose the cafeteria table with minor characters.

• Casanova: Your very first prom date! Let's take a pic of it!
  Successfully go with prom with one of the original six characters!
  
  
• True to yourself: You've been true to yourself and always chose the right option. Congratz!
  I would do a short single player game, and choose one of the characters' routes. Make sure to check your stats, and if need another guide or two to make sure your choosing the best answers!
  
  
• Good on my own
  I believe you can do this with the "Boo!" achievement, just choose to go to prom by yourself.
  
  
• Tiger Millonaire: You ended a run with 20 MONEY or more.
  Start with a regular single player game mode. Choose answers on the quiz that will give you MONEY, and visit the Library until you reach at least the 20+ MONEY mark. Then work on other stats to reach the love interest you want.
  
  
• Polyamorous: You've gone to prom with all 6 main love interests. What a Casanova!
  Successfully go to prom with all of the original characters, try both short and regular modes as these don't need to be secret endings for this achievement.

• Epic loser
  Get rejected for the first time
  
  
• Forever alone
  Go to prom with no one. I did this with "The Worst!", so your character will have a bad time at prom anyways.
  
  
• The best at failing
  Get rejected by the original six love interests. You might as well as get rejected by Zoe & Calculester (Second Term) as well.
  
  
• Broke
  Have negative MONEY by the end of the game. Easiest way to do this is a short single player game. Make sure you have high SMARTS and low CREATIVITY. Then buy the Corpse from the shop. When the first event choice happens with the zombie, choose to put it on your head and if your smarts are high enough you'll succeed. When the second event choice happens choose duel to the death, if your creativity is low enough you'll fail and lose 3 money.
  
  
• The worst!: You've been your WORST self. Congratulations?
  Choose all the wrong answers. The best way to get this achievement is when taking the quiz try to only get MONEY answers, and to go to the library during every turn. This will make sure that you don't have enough stats at the start to answer questions correctly, as in the game the answers start out easier and progressively get harder.

The items for these achievements will either be available at the beginning of the game, or will be unlocked with the more playthroughs you do.

• Perv!
  Buy the Tampon Used by the Former Prom Queen. There is a secret ending with this item for Vera, so you can also do that ending with this achievement.
  
  
• Boo!
  Buy the Blanket With Two Holes In It. This is a secret ending, so you must convince everyone that you are a ghost.
  
  
• Love from the depths
  Buy the Power Totem of Z'Gord, Ruler of the Dark Realms. This is a secret ending, so you can take Z'Gord (or Zoe) to prom. Note that this is only from the original game, not the DLC.
  
  
• Shopaholic
  Buy all of the items at least once.
  
  
• Enter The Coven
  Buy the Blood of an Angel. This is a secret ending, so you can take Joy to prom.
  
  
• Wowie!
  Buy the Set of Weirdly Specific Greeting Cards. This is a secret ending, so you can take Blobert to prom.
  
  
• 01101001 00100000 00111100 00110011 00100000 01110101
  Buy the Arcane Floppy Disk. This is a secret ending, so you can take Calculester to prom. Note that this is only from the original game, not the DLC.

These two achievements are unlocked after you play through the game. I believe that these unlocked after going to prom with the original six, and probably a few more playthroughs.

• I wanna buy... your heart
  This is a secret ending, so you can take the shopkeeper, also known as Valerie.
  
• Orgy manager
  This is a secret ending, so you can have a bit of "fun" with your classmates after prom.

Update to some achievements: Shout out to EndarkCuli!

FIRST TERM ACHIEVEMENTS

• Prank Masterz!: Buy the Eldritch Tome, and clear the corresponding route. As this involves Zoe's eldritch form, it can't be done in Second Term.

SECOND TERM ACHIEVEMENTS

• Great Commander; Hey, Listen!; Ultimate Trip: All of these require unlocking and buying items from the shop, and beating the corresponding routes. In order: the map, the fairy bottle, the drugs.
  
• Raise Your Hand Up: In the second event of the Ultimate Trip path, purposely fail the second event by having more Smarts than Creativity when asking Liam to raise his hand. This leads to Liam actually raising his hand, which happens nowhere else in the game.
  
• The one with the big reveal; The one with the evil twin: Buy the Lore Ticket and, respectively, take Hope and Faith's paths

GIFT-RELATED ACHIEVEMENTS:

• You Were Naughty!: Get Coal as a random item when picking the free Present from the shop
  
• Tiger Billionaire: End a run with at least 99 Money, which requires getting the Winning Lottery Ticket from the Present
  
• Cagatio: Get the little figurine from the Present, and complete the Damien/Liam plot line that follows
  
• The Funeral: Get the Mind Control tape from the Present, and fail at least two of the events involving Pheel the Eel

OTHER ACHIEVEMENTS

• Christmas Dinner: After receiving almost everything from the Present (route items as well as stat items, though you can just exit to the main menu after getting a prize instead of playing full runs), one more special item can now be generated from it. Get that item, and beat the corresponding secret route.
  
• Ultimate Lore: In the Hope path, have enough Bold (and more of it than Smart) to use the knife in the second part. This leads to the Coven talking about their past in great detail.
  
• Well Done, Player: After obtaining every other secret ending, the cafeteria..."opens up" a bit more (this will require a keyboard's arrows to find instead of just a mouse). Beat the corresponding route the new option unlocks. Regardless of failure or success, any attempt in a future run will also require purchasing a $15 top hat to re-select that cafeteria option.

Please know that this guide is INCOMPLETE, but I will fill it in with time, I just need to get the best way to help you guys get the achievement that is needed. Any comments to help complete this guide are appreciated and welcome!

I AM CURRENTLY NOT PLAYING THIS GAME, SO I AM NOT UP TO DATE WITH ACHIEVEMENTS AND/OR EVENTS. PLEASE SEE COMMENTS FROM OTHER PLAYERS IF YOU NEED HELP.

Thank you for viewing my guide!