F in the chat means to pay your respects. They are mostly on streams like twitch. The term originated after a feature in the 2014 release of Call of Duty asked players to press F on their keyboard or X on their controller to “pay respects” to virtual fallen soldiers.
A F in your grade means that you did good. You did so good that you deserve some ice cream with your parents. If they don’t believe you got a F, tell them you did, and once they see this definition, they will almost always give you what you want.