Soldier is bad with names so, he gave all a codename
The way Soldier comes up with each codename is with the way everyone behaves or the skills that they've shown so far.
Scout, he usually climbs whatever to have a better view to ensure it's safe for Soldier to move around the school (since he isn’t part of it).
Pyro, they already have that nickname, but Soldier knows it's because their fixation with fire.
Medic, the one that has the ambition and knowledge to be an actual medic in the future, he's usually the one patching his friends up or showing his vast knowledge about his future major.
Demo, he's Soldier's childhood friend, and he knows Demo is really into chemistry because of his parents. When playing videogames he always picked explosive weapons, hence the nickname.
Heavy, he's a big guy and Soldier has seen how truly strong he actually is, he's like a rock, unmovable, but a softie inside. He considers him the tank of the group.
Sniper, he loves causing playing with his slingshot, so one day Soldier saw what great aim he has with it and so he called him by that. What a compliment, and makes him sound badass.
Engineer (Engie), he's the smartest of the group, not only that but he's always seen carrying tools in his bags, welding goggles in his backpack and when loves spending time in the school's workshop.
Spy, he isn’t fond of the way Soldier calls because it sounds like an insult, but he's flattered in a way. He's great at gathering information, and if it is physical data like documents he WILL get his hands on them. Not only that but he's capable of other not so legal digital actions only if he really needs to go down to it.