Complex sentence: He attended The Calhoun School from age 3 through 8th grade, where his mother teaches science, until high school when he attended Bronx Science for one year before dropping out at the age of 15 and started homeschooling.
Split into simpler sentences: He attended The Calhoun School from age 3 through 8th grade, where his mother teaches science. He went on to attend Bronx Science for one year before dropping out at the age of 15 and started homeschooling.