When thinking that the local public school is otherwise equivalent to your boarding school, you should consider two things in addition to the things that others have noted:
What's the base rate? Generally, private schools are better than public schools. Otherwise, people would not pay for private schools. Anecdotally, I am always appalled by the things I hear about public schools (since my kids went to private school). I'm also appalled by my own memories of public schools. The qualitative difference is usually pretty big.
Public schools' reputations are usually overblown compared to private schools. Generally, you get into a public school by where you live. The reputation of the local public school thus affects property values, giving all the locals a strong incentive to claim that their school is good. Additionally, they feel more comfortable assuring themselves that the school that they are sending their kids to for free is a good school. That is, their conclusions that their schools are good is motivated reasoning. The result is that 75% of people believe that their kids' public school is above average, which is just impossible.
I went to the public middle school, which is the exact same group of kids, so I know a lot of students there and have talked to them about it. It genuinely is the exception to the rule that a public school can be almost as good as a private school (and better, when you throw in the commute and the snobbery involved with the private school). I went to school in a different district when I was younger and it was terrible, even though the school was considered really good, so I know exactly what you're talking about.
I don't know what you mean by "base rate", but people seem to do pretty well at both schools.
UPDATE 3/16/16: I decided to go to public school, because I was tired of all the little annoying stuff at my current school--especially the entitled kids and the entitled attitude in general. Everybody acts like they deserve something. It's very irritating.
The other reason I came to that decision was "exploration value". By moving to a new situation I learn whether I really am better off in the kind of environment offered by the public high school; even if it ends up being worse for me, at least I know what to avoid. If it's good, I know it's good; and if it makes no difference, I know that, too.
EDIT: Just to clarify, I attended public middle school with the same group of kids, so I won't have to worry too much about getting to know new people. I am still in occasional contact with my old friends. I talked to one of them for several hours just yesterday.
(I'm new here, though I've lurked, so if I break any rules or otherwise do something detrimental, please let me know and I will try to correct my mistake)
I currently go to a rather nice independent high school. I'm on significant financial aid, so I can afford it. The academics there are outstanding. However, being a boarding school (I go as a day student) it requests a lot of our free time. We are required to participate in adult-sanctioned activities at least six or seven hours a week, in addition to normal classes. This means that 1. I get home from school around 6pm and 2. (more importantly) it's very hard to socialize when you don't board at the school, and there's really very little besides drama or sports (neither of which I like very much) that people do after school and actually enjoy/make friends in.
I'm strongly considering transferring to my public school, which is unusually good for a public school, as a junior next year. The academics are not as great (classes are less discussion-based and there are not as many APs offered) but there is a strong amount of participation in stuff I might actually enjoy after school (math team, etc.) because we're not required to do anything after school. I've noticed that when people tell me I have to do things, I enjoy them much, much less. Also, I won't have a commute, which would be nice.
Everything else about the schools is more or less comparable.
I'm sure when I think about this decision I am biased in some way. I'm probably succumbing to the sunk cost fallacy (sometimes I think if I'd been at the public school the last two years, I'd rather be at the private school) or something like that. If not, I'm facing the problem of Buridan's Ass.
My question to you: should I transfer or not? I have thought very hard and consulted several intelligent people, and have not been able to come to any sort of conclusion.