The problem with going for the lime soda bottle rocket option is that it would mean he has to stop the flapping on the one thats filled with water. And its kinda sorta almost halfway there with the pressure building. That would be like a lot of wasted effort on his part. Plus he wont get to use his totally awesome bottle rocket.

Wrinkling his lips he looked around-- well most people in class were doing one form of the chicken dance or another...

Guh suck it up Eagle boy....

He flapped both arms now, wide and fast, not really watching if his long arms would take someone's eye out. The pressure within the bottle intensified greatly, tiny bubbles started to fill where the water touched the interior until..

PSSHHHHFFFFWWOOOOOMMM!!

Aaand he has lift off!