I am most familiar with Blair Academy, Peddie, and Mercersburg. I like all three schools even though three distinct environments.
Blair has some country club qualities, Mercersburg is friendly & caring, and Peddie has a strong athletics program.
With respect to PG programs, my approach would be to go to the school’s website and see if there are any PG only courses. Often schools have 2 to 4 PG only courses such as “PG English”.
Why do you want to attend a PG year ? Sports development ? Academic improvement ? Both ? Other reason ?
Some service academies have a list of designated schools for their admits who need to strengthen academics. The US Naval Academy PGs are among the best & most decent students that one will ever meet. I am not sure of the current list of PG schools for the US Naval Academy, but I will attempt a quick google search & report back.
From this list: Blair Academy, The Hill School, Kent School, Mercersburg Academy, Peddie School, Portsmouth Abbey, Western Reserve, & Wyoming Seminary are schools which I personally know that are all well respected. (Blair Academy & Wyoming Seminary are consistently the top 2 prep wrestling schools in the nation. Both have world class wrestlers. Wyoming Seminary is prominent for theatre, many sports, & strong academics.)
P.S. All schools on the US Naval Academy list should have strong athletic programs & solid academics. Of course, schools such as Andover, Exeter, Lawrenceville, Choate, & Hotchkiss all have stellar academics.
Really depends upon the student’s reasons for considering a PG year as to which are the most suitable prep schools.