You don't have to get 3 dresses! last year i went to two different ones and i got away with wearing the same dress but different accessories and shoes ;)
I like a darker lace one rather than a lighter colour for some reason. i think its ok to wear cocktail dresses at year 10 cos that's what heaps of people did at my yr10 formal :)
I don't think it would look plain if you had bright accessories with it like jewellery and stuff. since the shoes are red it would add pop into the outfit! and i think LBD are nice