My son declined an offer of admission from Groton as a 9th grader last year, but that doesn't qualify me as an expert. I am happy to share what I learned or divined through my experience.
If you are looking specifically for a school that takes 8th graders, I would start by searching on www.boardingschoolreview.com to identify your list. I'm not sure how comparable the ISEE percentiles are to the SSAT percentiles.
I would say the the interview is a huge part of the evaluation process. Many applicants, on paper, are interchangeable. It is their personalities and passions that separate them. There is a good bit of luck involved as well - get an admissions officer who your kid hits it off with, and your chances go up. Get one where the interests don't align and the chances go down.
My son was a recruited athlete, so having coaches pulling for a kid helps in a big way. If your kid's interest and accomplishments in computer science are attractive to that department, I'm sure that would help as well. If you feel that is a possibility, your kid should be reaching out to that department to set up a meeting while on campus, and stay actively engaged with that teacher right up through March 10.
My impressions, and these are nothing more than my impressions based on my limited experience, is that Groton likes fundamentally nice, thoughtful, wholesome kids. They are less preppy and aristocratic then some of their peer schools. While it is a religious school, religion of the applicant isn't important to them. They are less sporty than some of their peer schools. Because of their size, I believe they need and seek out kids who can fill multiple roles on campus.
Like all schools, Groton likes to protect their yield, so if after the conclusion of your interviews Groton is truly your child's number one choice, (s)he should reach back out to your admissions officer and convey that sentiment strongly.
Groton has a roughly 13% acceptance rate and 93rd percentile SSAT average. Groton is also very small, meaning that they need every kid to contribute in multiple areas in order to maintain the vibrancy of the campus. If your child would be an impact athlete there - exploit that. If not, I would stress the computer interest/ability and show he can contribute to Groton with that skill and take advantage of what they have to offer in that area. Your child should be taking the SSAT soon to get a handle of where he might fit in. Schools like Groton turn away hundreds of perfectly qualified kids every year. When looking at schools of Groton's caliber, don't get too enamored with one school, because unless you have a real hook, chances are you will end up disappointed.
the OP should go to www.boardingschoolreview.com where she can search for schools in the states she is interested in and further limit her search by schools with swimming programs, or whatever other search criteria is important to her. The site will then spit out a list of schools that match her criteria. It really is a wonderful tool. Once she has identified the schools she is most interested in and researched them on line, coming to a place like this for personal experiences and anecdotes might be helpful. Doing your search in the reverse order is counter-intuitive.
@AppleNotFar That can be a bit of a concern depending on the school. Ivy and NESCAC schools will require a minimum SAT for admissibility, and they don't want to screw around with someone who doesn't have the academic chops. They will work off PSAT scores initially. For NESCAC, the timeline isn't THAT important, as they cant take a commitment until July 1 following the junior year of high school. For Ivies in sports that engage in early recruiting, you are often committing to the admission process, which means you still might not get in if all the academics don't line up. No, athletes don't need to take SAT's as a sophomore or even fall of junior year to be competitive in the recruiting market, but yes, a student looking at Ivies and NESCAC schools can disadvantage themselves if they wait until very late in their junior year to take the SAT. Recruiting is a meat market, and if you don't have all your stuff lined up and ready to go they will move on to the next kid who does.