Well, many teaching certificates require a special endorsement to be a School Psychologist, but to teach the subject at a High School without being the School Psychologist, I think most schools just require a regular teaching certificate.
If you go to this site http://www.uky.edu/Education/TEP/usacert.html, there is a drop-down menu where you can choose a state and get the certification requirements for your state.
If your state requires additional work to teach the Psychology class, it will be listed in the certification requirements... but where it lists additional requirements, it will usually be for School Counselors, so read carefully.