Visit the schools, they are all good but might not be a good fit for your child.
Groton, St Marks, Rivers in that order for academics--Groton is a top 12 school in country and super competitive to get in; St Marks and Rivers little easier go on boardingschoolreview.com for SSAT, acceptance rates, endowment etc...Hockey is different story. Rivers has great hockey and regularly makes it in NEPSAC tourney. St Marks good hockey...Groton is hit or miss although last year they did well and heard they are trying to get better with recruiting new players. End of day poster is correct with fit being most important,(and $$$ package) as if your kid needs to play and if he's a stud he will play d1 after a year or 2 in USHL, BCHL, or NAHL or go right to NESCAC if lucky and had grades...
Honestly it really depends on what you are looking for. In my opinion Groton has the best academics if your child can manage the workload. On the hockey front I would give the edge to Rivers and St. Mark's. However Groton did surprisingly well last year and I hear they have another great recruiting class joining them this year. St, Mark's is your all around school, good academics and decent hockey, I also hear they've had a good recruiting class and should be the better of all 3 teams. Rivers is a step above HS, not sure why you would have it on your list unless you are looking for a hockey school, if that is the case go to Dexter, St. Sebastian, Salisbury or Kimball.
I would argue Groton is top 3 in Academics in the country.