Does admission officers look at 12th-grade grades?

All UC schools look at 12-grade grades. And the harder ones UCLA, UC Berkeley scrutinize them more than the others.

First of all, the UC system is its own animal and they have a super early RD application deadline compared to other schools. Nov.1-30 is nearly 2 months earlier than other colleges. If they do not immediately look at 12th-grade grades it's not because they don't care but because they are not available until the Mid-year report in late Jan/early Feb., so all they can look at are quarterlies or trimester grades if a school reports those.

My advice to you is to take the most appropriate courses to keep your GPA as high as possible and show evidence of your course rigor.

Your success or failure is more dependent on what you did during 9-11th because all colleges don't have a clear picture of 12th grade. But if you are an A student with a 4.0 GPA of 9-11th then they will think you are going to be one during 12th. I don't think they will wait to decide to admit you until your 12th grade GPA comes out unless you were wait-listed, then they will make sure you deserve the spot over someone that didn't work very hard during 12th grade.

Remember a couple of germane facts:

1.) More HS students are applying to UCs now that all standardized tests are optional.

2.) UC schools are getting a record-breaking number of applicants.

3.) All the previous admit rates are no longer a good indicator of what future admit rates are going to be. (BUT they are going to be much lower therefore there will be more competition all across the UC campus. UC Davis might see admit-rates in the high 20s and UCI might see them in the low 20s. And I can see that UCLA might go below 10% and UCB about 12% this year

Therefore err on the side of being over-prepared.

Good luck.