Complex sentence: The old high school building now houses Southaven Middle School, serving 6th through 8th grades, and the former middle school building now houses Southaven Intermediate School, serving 3rd though 5th grades.
Split into simpler sentences: The old high school building now houses Southaven Middle School, serving 6th through 8th grades. The former middle school building now houses Southaven Intermediate School, serving 3rd though 5th grades.