do colleges mostly look at junior year?

Answered

Top Colleges like Stanford mostly consider your 10th and 11th grades equally. They care less about 9th grade and if you apply SCREA to Stanford, they will re-calc your Stanford GPA based on your 10th/11th grades only. If you apply RD, then they will look at your 1st semester 12th grades.

One reason 11th grade is critical is because it is the last full year of grades/course rigor/testing that can advocate for your academic narrative. Most high achieving students max out their APs or IBs during their 11th grade as well as their ACT/SAT testing. Typically for high achieving students 11th grade marks the pinnacle for your HS narrative because in 12th grade, you only get to show about 1/2 your potential since college applications and results come back a couple months prior to graduation and your final AP test scores. Plus there are senior class activities like prom, awards, filling of college applications, getting recommendations, writing essays. These cut into your senior year potential and progress.

Most colleges will look at all your transcripts from 9th to 12th but historically the best student complete their best work during 10th and 11th grades because they have less distractions. 12th grade is really to tweak your already establish narrative and make adjustments to your ECs and decide where to apply to and organize all your thoughts about completing the college admissions process.

I hope this addresses your question and answers it to your satisfaction.

Good luck.