Complex sentence: He continued to study sculpture throughout high school, his progress prompting the art director for the school district to lobby the school district pay to send Stevens to evening classes with Harvard students at Copley Plaza Studios.
Split into simpler sentences: He continued to study sculpture throughout high school, his progress prompting the art director for the school district to lobby the school district pay for Stevens to attend professional off-campus classes. Among these were evening classes with Harvard students at Copley Plaza Studios.