When I toured Grace, they said most of their 5th graders end up at SSSAS. At that time they only had 5 or so kids in the 5th grade. It gets super tiny.
For K-8 schools, the Catholic schools will send most of their kids to one of the Catholic high schools. ACDS says they generally have no issues getting their kids into their choice high schools. They send kids to SSSAS, Episcopal, Ireton, Visitation, Gonzaga, Madeira, etc. Burgundy Farm and Browne are also options that end at 8th. They'll generally send their kids to the same as above or public. Kids in Fairfax County often have better public high school options, including TJ.