so you want a bike you can ride some distance(to school) and do flatland on it and you need to be able to leave it outside, locked up?
most flat specific bikes have gearing that is so low you can't ride anywhere on them. but if your just learning, you don't need that specific of a bike.
I would go w/ a complete bike from some company like DK, Haro, Eastern or something like that. they will all be about the same quality and if your just starting out it really doesn't matter what you get.
by the time you have some decent skills you will know what you want from a bike. for now just go with a low to mid priced bike and have fun.
shorter frames are easier to learn flatland tricks on, so keep that in mind. I would go w/ a 20 inch top tube.
it's longer than most kids use for flat but it will allow you to learn basics and still have enough room between the bars and the cranks/seat that you can ride down the street w/ out feeling too cramped.