Answer:
The comma should be place between the words "grade" and "are" as follows: "Kari and Misha, who have known each other since first grade, are both yearbook editors and photographers".
Step-by-step explanation:
One of the uses that comma has is as parenthetical punctuation, separating an idea that serves as additional information to the main idea of the text. The correct way to use it is separating the additional idea between commas, just like a parenthesis. In the example the information in the text "who have known each other since first grade" is additional to the main idea of Kari and Mashi being both yearbook editors and photographers, therefore it should be punctuated between commas.