Clear Lake Union High School opened in October 1901, in the old Academy School building in Lakeport, California. F.G. Sanderson was the founding principal, and there were 40 students that first year. Mary Stipp, who had begun at the Academy, was the first graduate in June 1903. By June of 1904, there were fifteen graduating seniors. By 1917, the enrollment had grown to 111 students, taught by seven teachers, and graduated 25 seniors.

In 1916, a lot was purchased by the people of Lakeport and a fine new home for the school was erected noted for its elegance, convenience and comfort. The school site bordered Clear Lake and was located where the present day Del Lago townhouses are situated.

Clear Lake Union High School became Clear Lake High School when the elementary and high school districts unified July 1, 1963. The school moved to its present site in 1965 and currently has an enrolllment of 470 students with 24 full time and 3 part time teachers.