Complex sentence: By 1854, the system had 27 schools and served nearly 4,000 students, in 1855 a high school was opened with considerable fanfare, and by 1860, nearly 12,000 students had enrolled in the district.
Split into simpler sentences: By 1854, the system had 27 schools and served nearly 4,000 students, and in 1855, the district opened a high school to considerable fanfare. By 1860, nearly 12,000 students had enrolled in the district.