The new people in Gravity Falls (Assassination Classroom x Gravity Falls)

This idea popped in my head when I was about to go to sleep. Gravity Falls highschool. Also, I'm sorry the terrible description.
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Koro Sensei is still alive okay! Bill Cipher is human/demon and a teen.
KarmaxNagisa
IsogaixMaehara
BillxDipper
Karma teasing almost everyone in their class.
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Karma, Nagisa, Maehara and Isogai are told they are needed in Gravity Falls, Oregon. But why? They need to watch a teacher for the whole time and assassinate him before graduation. Karma, Nagisa, Isogai and Maehara are already pro assassin's. There is one problem, Bill Cipher, Dipper and Mabel and the grunkles. They are suspicious of them so they try and stay clear...