California doesn't organize by county so it's hard to tell. Riverside County has Great Oak, Vista Murrieta, Eleanor Roosevelt, Murrieta Mesa, and MLK Jr that have all produced some phenomenal teams in the past couple years. I am only going to include sub-1:56, sub-4:20, and sub-9:20 but there are plenty of runners close to those times. In 2018 there was:
Carlos Carvajal - 9:05 3200 and 4:13 1600
Tyler Tickner - 9:15 3200 and 4:19 1600
Gabriel Abbes - 9:16 3200
Gavin Korby - 9:18 3200 and 4:15 1600
Chris Verdugo - 4:17 1600
Edgard Villa - 9:14 3200 and 4:14 1600
Willie Coleman - 1:54 800
Jared Angle - 1:55 800
Raymon Ornelas - 9:08 3200 and 4:15 1600
Jackson Taylor - 1:53 800
All of this in one year, many of these runners not being seniors. However, considering right now is kind of a glory year for Mercer County, we might as well compare to 2016, which was Riverside County's glory year. That year, there was:
Tyler Janes - 8:56 3200 and 4:04 1600
Andrew Boebinger - 4:16 1600 and 1:55 800
Michael Norman (only sprinter I'm including) - 10.27, 20.06, and 45.51...now runs 43.4
Raymon Ornelas - 9:08 3200 (only a sophomore at the time)
Isaac Cortes - 1:50 800, 4:04 1600, 9:00 3200
Spencer Dodds - 1:54 800, 4:05 1600, 8:55 3200
Cole Spencer - 4:08 1600, 8:51 3200
Solomon Fountain - 1:55 800, 4:10 1600, 9:19 3200
Nelson Quintana - 1:50 800, 4:16 1600
Tony Robinson - 4:18 1600, 9:18 3200
Ryan Jones - 4:19 1600
Carlos Carvajal - 9:16 3200 (only a sophomore)
Amazingly, every person from Isaac Cortes on was from Great Oak. Anytime a Loudoun Valley fanboy comes on here, direct them to this list...and then tell them that Great Oak is not even the greatest high school distance team of all-time (that honor, imo, belongs to 2017 American Fork).
It should also be noted that I am only looking at a few select schools, because I don't know every school in Riverside County and frankly can't be bothered to look them up. I am sure there are other runners at this level that I missed. The point is that Mercer County is excellent, especially in 2018, but there are other counties with a similar density of high quality runners across the country (I won't give any proof of this but I'm pretty sure there are a bunch of really good teams in a small area near The Woodlands, Texas that have put together these kind of numbers as well).