Help me gain weight fast and lose it fast too?

i am a serious dancer, but my dance coaches have stopped me dancing because i am too thin. i am 5 foot 6 and a half and weigh about 7 and a half stone at the moment ( i have gained about 4 lbs reacently - but dont know if thats mass or actual weight ..) but i am only allowed to dance again when i weigh a whopping 8 stone!! how can i quickly gain half a stone.. be able to prove to them i weigh 8 stone.. and then just as quickly lose the weight so i get back down to 7 stone 11. i think this is a good weight for me and would be happy at 7,11. but i need to be 8 before i can dance. thankss