I heard that certain colleges disregard your academic performance in 9th grade and just look at 10th and 11th grade. Is this true? If so, which colleges do it?
Most colleges care much less about your freshman grades than your sophomore and junior year grades, but your performance freshman year will still affect your final high school GPA (which they do care about). If your GPA freshman year was particularly low, this will be evident in your final GPA even if you did well sophomore and junior year. However, in a case where you had a slightly lower GPA your freshman year but did incredibly well for the rest of high school (for example, your GPA freshman year was a 3.6 but you had a 4.0 sophomore and junior year), you probably wouldn't be penalized in the admissions process.
Few colleges explicitly state that they don't look at freshman grades. The only ones that I've come across are the schools in the University of California and California State University systems, McGill University, and Stanford University. Stanford will see your entire transcript, but they say "We will focus our evaluation on your coursework and performance in 10th, 11th and 12th grades, primarily in the core academic subjects of English, mathematics, science, foreign language and history/social studies."
Even at these schools, admissions officers will still review your class rank, which will inevitably be impacted by your freshman grades. It's best to start off strong even if your grades freshman year matter less than your performance in 10th and 11th grade. For more information on this topic, check out our article on which year of high school is most important.