Lost Judgment

There are a total of 8 Skateboards in this game plus one dlc board but that isn't required for this achievement. Here's the name and how to get all 8 Skateboards as shown in order of how they appear after collecting them. Some boards will require you to buy them with money. Some boards will require points you get from skateboarding which will require you to progress through the School Diary side story to the point to get to unlock the skater's activity, and once you start it the store and the missions to get lots of points can be found on the east side of Hamakita Park as a green marker called Skate Park. Heaven's Door can only be unlocked through the Casino activity in School Diary which is pretty late into that side story but once you unlock the casino you can play Poker or Blackjack and make chips fast if you have the cheat items for them (Would suggest having a few hundred thousand Yen to get a few thousand points to start out with and to save before gambling).

Blue Tribal - Free - From Sugiura (Story)
Yellow Lizard - 20,000 pts - Skate Park (Ijincho)
Green Lizard - 50,000 Yen - Ebisu Pawn (Kamurocho)
Skull Bass - 50,000 Yen - Deco Rush (Ijincho)
White Horse - 50,000 pts - Skate Park (Ijincho)
Red Horse - 50,000 Yen - The Bee (Ijincho)
Purple Drop - 100,000 pts - Skate Park (Ijincho)
Black Drop - 36,500 chips - Heaven's Door (Ijincho)