I can give you some first-hand information on El Camino.
I went there from 1989-1993. I knew the people on the champion teams. The teachers are all unbashedly anti-Republican, and brainwash the students as such. As a result, for example, in a mock election of 1992, Bush Sr. came in third behind Perot. I recall, one Academic Team member, made some long-winded speech in class with tears about how their health coverage, by way of their parents insurance, cuts off for college students past the age of 21
at the time.
My U.S. history teacher, the head coach of the Academic Decathlon team at the time, openly displayed a Clinton / Gore campaign poster. Other non-history teachers put up political cartoons by Conrad of the LA Slimes (Dan Quayle saying "Don't forget to vot.") My English teacher would repeated bash Republicans on their anti-abortion stance.
My cousins go there now and the enviroment has changed little. The same U.S. history, now just a coach of the Academic Decathlon team, took down his U.S. flag and replaced it with a tiny one you find on tables when OIF started. When Bush was re-elected, he threw it in the trash can and called Americans in red states stupid. He would proclaim Kerry the most intelligent candidate to run for President since Carter. Never once admitting that Kerry didn't get into graduate school and Bush did.
The sad thing is some of those smart students do go off to liberal colleges to continue their imbalanced promotion of selective thinking when it comes to politics.
I also thought that the other "reason" given was laughable: That the poster promoted one ideology over another.