Twilight Sparkle: "Ya rainbow ankle-biter!" "That'll slow yer down, ya twitchy hooligan!" "Ya miniature delinquent! "Couldn't skip around that could ya, ya precious little pony!" "Ya spastic little gremlin!"
Well if Rainbow Dash was allowed to use her wings in the Iron Pony competition, and no one called foul on that, then I see no reason why Twilight shouldn't be able to use magic. It's poetic justice from that episode, and the first time Twilight met Rainbow.