Lip-synching skits are pretty much a no-no. They get boring and very awkward. I would recommend writing a skit with your own dialogue. It's more work, but the audience and judges will appreciate it a lot more.
Conventions for 2015 (so far...): Momocon, A-Kon, Otakuthon, Sac Anime
Cosplay line-up: Mayoi Hachikuji (Bakemonogatari), Nona (Death Parade), Nagisa Momoe (Madoka Magica), Menma (AnoHana), Tsukihi Araragi (Bakemonogatari)