We live near downtown Bethesda and our kids, like many others we know, attended the Bethesda Montessori School for 3 years (incl K) and then went into first grade at Bethesda Elementary (public, MCPS). My kids still remember their preschool fondly. They learned so much there. All the Kindergartners could read well by the time they graduated.
Public was a lesson in socializing and dealing with large classes and noise. The academic content would have been easy whether they'd attended a public or a private, because primary schools always need to focus on the least advanced students, since reading and counting are so essential to what happens later. Most of the Montessori kids we know later ended up in magnet programs, advanced tracks in their public home schools, or one of the Big 3 privates.
My kids are adults and teens now. Youngest is at Bethesda-Chevy Chase high school, doing practically all AP courses.