This probably isn't the answer you're looking for, but in my opinion: it depends.
It depends on your personal situation, the attitude among your friends and parents, the environment at school, and the prevailing political climate in your area. But mostly, it depends on how comfortable you are with coming out.
This is a decision that has to be reached individually, each time someone is faced with these issues. My situation was different than yours, and yours is different than that of another gay person coming to terms with their sexuality in your very same high school. The key, in my opinion, is to try and not pay attention to the pressure from either side of the issue, either the pressure to come out or the pressure to stay under the radar. It's your decision, and only you can decide when the time is right.
The other thing I want to point out is that there is never any end to the coming-out process. Every time you find yourself in a new situation, a new job or a new school or a new group of friends, you'll be faced with this exact same question. Who should know? How much should they know? How do I tell them? So basically, there'll be no shortage of coming-out opportunities in your lifetime.
It seems to me that you're handling it well so far. I did the same thing in high school, telling only a few select people, and it worked out pretty well for me. I do wish I'd told my mom earlier, but I'm not sure it would've worked out any better. The end result, that I'm alive and healthy and I have a wonderful boyfriend with whom I'm very much in love, couldn't be any better, so I'd say I did pretty well.
If you don't feel safe doing so, don't come out. On the flip side, if you feel that keeping your secret is harming your relationship with someone, that's a major factor to consider when deciding. One serious rule is: if you think your parents will kick you out of the house, don't come out. It's estimated that half or more of the homeless teens in the U.S. are gay, and you really don't want to join that demographic.
The inevitable informative links follow:
The GLSEN on coming out:
http://www.glsen.org/cgi-bin/iowa/all/library/record/1290.html
PFLAG on coming out:
http://www.pflag.org/For_GLBT_Persons.comingout_glbt.0.html
Believe it or not, Wikipedia on coming out:
http://en.wikipedia.org/wiki/Coming_out
I hope you find this helpful. There are a lot of great resources out there, including these boards. Keep asking questions, we love this sort of thing.