You're planning ahead a lot more than most kids that move to SF for a summer. Most end up homeless on haight street before calling their parents for a ticket home (I've hosted two separate street kids who moved here with starry eyes and ended up on the street before they went home...) -- so kudos for planning ahead a bit more than them :)
I moved here with $7,000 + a $2,000/month contract gig and settled in nicely. You're cutting it close, but it's possible. My $0.02x4:
(a) It's possible once you have the network and people who will vouch for your skills. Be aware that CL is full of red herrings; people are not hiring as many contractors in the downturn, but it still happens. Expect to see far less C++ than RoR/python/PHP.
(b) It will be challenging, mentally and physically, but it's possible if you're strategic about it. $700 can get you a sublet __if_you_look_long_and_hard__, but $100 for food is tough. You probably won't have much of a kitchen to prepare your own food. When I went through YCombinator I lived off of Vietnamese sandwiches and Dim Sum for days at a time; you'll get sick of it but $3 sandwiches and $4 for a plate of dim sum is hard to beat. $7 mission burritos are also a good bet, El Farolito's has burritos that'll easily feed you for a whole day.
c) SF is going to be cold during the summer. You should have been able to find this out, it's no longer 1968 ;)
e) You're in high school, you get to be crazy. :)