I agree with @preppedparent - read the view book - its also a good idea to look at the course catalog on line, look at the upper level/AP classes so you and/or your child can reference whatever opportunity may exist that catches his or her eye.
As someone who was in a similar position to yours at this time last year, resist the urge to feel like your circumstance is unique. To the contrary, there are hundreds just like you, coming from an under performing public school where everything is too easy, looking for the child to be pushed, rub elbows with equally motivated peers, etc., etc., etc.
That is BORING! Don’t be boring, be interesting. While your circumstances are not, unique, your child is - figure out how to show that to your admissions officer
Are you full pay? from an underrepresented geographic area? a minority? The one thing your child’s profile is light on is community service/extra-curriculars. Schools expect to see that.
Good luck in your pursuit of Groton - it really is an amazing community. I know my son would have been very happy there had he chosen to accept their offer.