Your online presence should be a reflection of your school in its entirety - and no school is perfect. You should handle negative reviews the same way you'd handle a disgruntled parent/employee on school grounds. Acknowledge their feelings, address their concerns and offer a solution.
If your page doesn't have very many reviews to begin with, the bad ones can really stand out - even when addressed appropriately. Ask five super-engaged parents who you know love your school to go onto Facebook and fill out a review - then, when prospective families find you, they'll get a balanced picture.