Some cross country fans try to keep track of this sort of thing. I'm happy to help where I can. So, here is a list of top ten teams from each classification that sat out opening weekend (unless they slipped unnoticed out of state):
5A Boys - Monarch, Rampart
5A Girls - Broomfield, Monarch, Fossil Ridge, Fairview
4A Boys - Centaurus
4A Girls - Centaurus, Golden, Evergreen
3A Boys - The Classical Academy, Salida
3A Girls - The Classical Academy, Steamboat Springs, Salida
2A Boys - Lyons, Heritage Christian Academy, Sargent
2A Girls - Lyons, Nederland, Crested Butte, Heritage Christian Academy, Colorado Springs School
Most, if not all, of these teams will be in action this weekend.
The UCCS Rust Buster picks up the Colorado Springs School, Rampart, and The Classical Academy. The Rocky Ford Melon Buster picks up Sargent. The Arapahoe Warrior picks up Fairview, Golden, Salida, and Steamboat Springs. Centaurus picks up Broomfield and Monarch. Lake County picks up Crested Butte. CSU/Frontier Academy picks up Fossil Ridge and Lyons. The Ponderosa Mustang Stampede snares Evergreen.
So far, I've not pinned down a meet for Golden, Heritage Christian, and Nederland for the coming weekend, but that does not necessarily mean they aren't competing.
In all, based on what we see above, I'd guess that about two-thirds of Colorado's high school cross country teams were in competition on opening weekend. Of course, several individuals were impacted by the Saturday administration of the SAT exam.