I come from a very competitive private high school and there is a lot of competition to be the top of class compared to a local public school. Is being in the top 25% of a tough private school as good as being in the top 10% at a public school?
Admission officers are well aware that not all high schools are equally rigorous. They realize that at challenging schools like yours, the workload can be heavy and good grades are hard to attain. Therefore, the admission officers typically are more generous to applicants to hail from the most competitive high schools.
You will be evaluated in the context of your own school community and will not be compared head-to-head with the public school candidate from other areas.