This actually started life as a 4-panel duel between Eric and Charles, with this being the punchline after Xavier's claim that Magneto's powers could not affect him. The concept tickled me, but it felt like there was too much waffle. As I started trimming things down, I suddenly thought "why not just the last panel on its own?"
Better still, in changing the motive to be a prank it just made everything more light-hearted and friendly, which appealed to me so much more. There's something quaint about the thought of Magneto hiding in the bushes and chuckling to himself like a child.