Complex sentence: Niagara Falls High School is operated under the supervision of the Niagara Falls City School District and is the only high school in the district, since the merger of the "old" Niagara Falls High School and LaSalle High School.
Split into simpler sentences: Niagara Falls High School is operated under the supervision of the Niagara Falls City School District. It is the only high school in the district since the merger of the "old" Niagara Falls High School and LaSalle High School.