Here's an idea I just sketched out. If you want to do the "cookie" idea, of shooting a flash through plants, you'd place the plants immediately to the right of the panel, and use a third flash for that, to cast the shadow on the background. The video above shows how that would work. The diffusion panel is BIG, as in door-sized. This is my standard old Lightform size, and with speedlights, you'd want two flashes for even floor-to-ceilingt fill light, to show the dresses and shoes and get adequate light on the couples. The thing is, in a confined space, you NEED FILL light at the bottom of the couples, at least a little. With the main light fairly close, and casting a modified loop light type of shadow pattern on the faces, the big danger is really BRIGHT faces, and then the entire bottom of the frame looking VERY dark. I see this a LOT in many speedlight flash set-ups, where the faces are hot, but the belt line area is 3 stops under, and the floor is -5 stops--and that looks, in a word, crappy.
This is not "portraiture" as much as it is "catalog fashion" with prom-goers as the subjects. You want to show the gowns and dresses and hair, the whole outfit, and you have (usually) two people, so having the fill light going from floor-to-ceiling is going to be nice, and make the shots look better.
If you happen to have a BIG strip box, you could use that. If you only have one speedlight to use for the fill, I would put it in an umbrella, camera right.
The PANEL does something you might not be aware of:
it gives the subjects PRIVACY. Keep the other kids to the right, and away from you. DO NOT let groups of others hang out behind your shooting position. Keep them AWAY, so the couples can react to YOU and the camera.