NO, incorrect. Today, you act for the French teacher in the third grade. Today, you are standing in for the third grade French teacher. (A bit more British) Today, you are filling in for the third grade French teacher. (More American)

I'm a native English speaker. "Fill in" is probably the best usage out of those three. "Stand in" is probably fine, too. "Act for" is really weird in that context. If you're "filling in" for someone, you're filling their shoes or taking on their role. You're doing their job in place of them. "Standing in" is more temporary and less complete. Metaphorically, you're just standing there. You might be a warm body, able to watch the class, but there are fewer connotations that you're going to do the entire job. "Act for" ... I haven't heard that very much. When people talk about 'acting' I think of plays, where you're acting as a fictional character. (Come to think of it, "stand in" is often used more in the context of plays, too. Like a stand in for someone to read your script with.) Hope that helps! Let me know if you still have questions!

Whoops, I put this answer in the wrong spot! I'm sorry!