What degree / level of higher education do I need to achieve if I want to teach high school sex ed?

You will need a Bachelor of Education. That will be at least 4-5 years. Depending on what program you go into. There are Bachelor of Education programs that are direct from high school and are 4-5 years in length; or you can take a Bachelor of Arts for 3-4 years in whatever subject you prefer as long as it is "teachable" and follow up with 1 year Bachelor of Education degree.