My Co-worker's Wild Child

My co-worker has twin boys who are just starting 8th grade. She is really cool and funny, and I imagine her sons are sources of endless hilarity.

She posted this on FB today, with the caption: "His first writing assignment in 8th grade: 'Write something that comes from within you.' Then later, with the comment, "...we figured it out. He's got an "F" within him."

I don't know which twin this is talking about.