Im not quite sure if this is the right subsection to post this in, I have already been warned for posting in the wrong section 
but what are some schools that are similar to Mercersburg? Preferably schools that have a good cultural diversity, believe in a strong sense of student body/community, have rigorous academics similar to Mercersburg, and is in about a 3 hour driving distance from Baltimore. I know its a lot of specifics but if you have schools in mind that apply to just the location, and would be considered a pretty good school, please mention it. Thank you!
There are all sorts of ways to search for schools. Gender, sport, location. As for your stated criteria: that’s all reputable boarding schools to varying degrees.
Blair, St. Andrew’s, Peddie, George, Hill School, Lawrenceville. I think that those fit your criteria for distance. I can’t speak to cultural diversity and strong community, but they are all well-respected academically. Look at their websites and Boarding School Review. Also this website: http://www.midatlanticboarding.com.
Hill! But I’m prejudice.
Episcopal High School in Alexandria, VA. Approximately 1 hr from Baltimore. EHS’s 100% boarding creates intense sense of community. Pristine academic, art, athletic facilities. MB/EHS are similar, yet different. EHS has Washington Program which is one of the major value adds. DD was fortunate to be accepted to both MB and EHS, choose EHS. Here’s a link to their website which can answer some of your above questions:
http://www.episcopalhighschool.org
Best of luck.
@19HSHelp An hour or so northeast of Baltimore, opposite direction as going to Episcopal, is St. Andrew’s in Middletown, Delaware. The school shares characteristics of both Mercersburg and Episcopal. Which is to say, 100% boarding with intense community feel, but not as isolated as Mercersburg, as you are so close to Wilmington (half hour) and Baltimore/Philadelphia (hour and change). Middletown itself has grown by leaps and bounds in the last ten years, so the pastoral, wooded campus of 2100 acres (surrounding large Noxontown Pond) actually is within walking and driving distance of many amenities that students appreciate. I have one graduating this year and a second next year. The school is on the small side, at 300, while very academic schools such as Peddie (550) and Lawrenceville (850) are larger, surrounded by suburbia, and barely within your three hour radius from Baltimore (but in the same athletic conference, the Mid-Atlantic Prep League, as Mercersburg). St. James is another much smaller school (220 or so) that is closer to Baltimore than Mercersburg, with far fewer financial resources than the others mentioned here, but I know a young doctor who has done well coming out of the school. Feel free to pm me if you’d like more on St. Andrew’s (SAS) after looking into it.
@19HSHelp I will definitely put in a plug for Peddie. We are very familiar with it (don’t want to reveal too much) and the one thing that has remained consistent with Peddie is its embrace of cultural and socioeconomic diversity. The school also places a premium on producing nice kids. I have also appreciated the fact that the school celebrates all senior college choices from “no-name” to the Ivies. Great culture.
George was the first one that came to mind looking at your list of priorities, but all the schools that people have suggested are great.