It's really tricky, it seems I'm the second one to obtain this achievement after MIX_UP_GAMERS. What I did was I got 4-5 questions wrong on the first notebook and ONLY solving the first notebook so Viktor can go faster at just the right speed, then I immediately got the soda from the punishment room. Then I chased the head teacher (walking while she's looking or might be looking, and running when she's out of sight) either to the corridors past Mr. Mix's room, or the corridors that lead to the black key shortcut and the courtyard, depending on which direction she floats to. Because she's faster than you, you just have to keep following her in the longer faraway corridors until you see her turning around and floating back to the punishment room corridor, but don't follow her back there. Instead, what you do is when you see her turning around after patiently chasing her through the very long twists of corridors, right when she's about to pass you, start running to break the school rules so she carries you to the punishment room. Hopefully, Viktor is standing near the MIDDLE of the corridors by the time you spot him, rather than near the walls, 'cuz that won't work if he is too close to the walls, as the teacher carries you while staying aligned with the middle of the corridors. Immediately equip the soda while being carried, and you must spill the soda on Viktor on the exact timing that you get close enough to him, the exact angle of the screen at which you open the can, and have Viktor at the closest distance to the middle of the hallway he is in. Those three factors are essential to obtaining this achievement, and may require a bit of practice to master them all. You'll know when you did it right when you hear Viktor bitching at you, and when the Achievement Get text pops out of nowhere in the middle of the screen.
Guess you're the third one. I tried doing the same thing with the chocolate muffin to see if there is also a hidden achievement for that as well, but the muffin always vanishes when I use it while being carried by h-teacher, you can't even see it being thrown, so I guess there's no achievement for that.
