Do colleges/state universities limit admissions from specific high schools?

I am wondering about how much impact your high school has on the admissions process. I know that HS guidance counselors can have good relations with certain college admissions depts. On the other side, I have heard that if there are a lot of kids from one HS applying to a college, it can make it tougher to get in since they will not want a ton of kids from one school. Even back in my high school days that idea was floating around, that only 1 kid from my competitive public high school each year would get into Yale.

My S is at a public school in CA and when I look at the school’s naviance, it seems like a large number of kids apply to many of the UCs every year and pretty much the same percentage get in every year. Are there invisible quotas for in-state high schools? How much does the applicant’s school get considered in the admissions process generally? Are some kids better served by applying to colleges that have fewer applications from that high school or city?

I think you already answered your own question. It makes sense that no school can fill its class from the amazing talent at any one school or any particular background. From the little I’ve seen, applications are grouped in various ways and then sifted for the best fit for that school for that year. The grouping could be by interest, by educational background, by sport or other talent, by geography or whatever the school finds important at the moment–or whatever category happens to come through the doors that year. Maybe sixteen unicyclists apply! You never know. The best unicyclist may go there, unless Harvard snatches him or her up. There may be groupings that answer to a specific scholarship that’s available, so that they award it properly. Or there may be a new professor or lab that is eager for students, pulling in more for that year in that subject area. But in general, a school needs to create a class that works for that school. No single high school or type of student will fill the entire entering class.

UCs do not specifically consider class rank, but they do consider achievement in context with background and opportunities. Within your high school, GPA, which UCs consider heavily, obvious does correlate to class rank.

Note that UCs do not weigh SAT/ACT scores very heavily, so do not overestimate your chances due to having high SAT/ACT scores.

Some other schools, like Texas public universities, explicitly use class rank.

Honestly, in all likelihood your school simply wasn’t as competitive as you thought. Since you have a son applying, my high school days were obviously closer to today than yours (although still >10 years ago) and ~45% of my class went to the Ivy League alone. That kind of stat is incredibly unusual for a high school, your kind of stat is more typical.

Setting aside the concept of “what opportunities did the student have available to them and how rigorous is their academic record” absolutely miniscule if at all. I don’t think 2 kids from two different well to do high schools are in any appreciably better shape than 2 kids from the same well to do high school.