I realize most CA high schools won’t give a class ranking. But a top student should know if they are top 1%, 2%, 5% etc and can use the school’s own profile to validate that (adjusting for major and relative strength of ECs as appropriate). What’s important is that the UCs compare all applicants from a given school and most strong instate students apply.
So it’s pretty predictable that if (say) 10 are admitted to UCB/UCLA each year then a kid in the top 15 students will have a good shot, assuming they are applying for L&S and have above average ECs. Conversely a kid outside the top 20-25 is going to struggle.