...about it. I'm a California teacher...and I'll share what I know.
First, each state will have a different curriculum. This is the website to find it for California:
http://www.cde.ca.gov/ci/ There is also a federal education department headed up by Margaret Spellings (Sec. of Education) you should check out.
http://www.ed.gov/index.jhtmlSecond, any curriculum probably won't be called 'sex education.'
Third, any curriculum will be developmental...which means the student learns what is appropriate for the questions they're likely to ask at a certain age. So, if a program begins in kindergarten, it might just teach a social skill (respect, for example).
Four, most programs have some way of parents opting out...usually a signed permission slip. So if a parent disapproves, their child is exempt.
Five, in my opinion, this whole discussion is based on a false assumption.
Many conservatives have (for ages) complained that prayer should be allowed in schools...saying that schools are terrible because they don't allow prayer. That has always been false. It's the teacher or an administrator MAKING a child experience an educator led prayer that is prohibited.
This 'dust-up' over sex education in kindergarten is the same idea. It's being spun as 'Obama promotes some high-age-level program about sex to 5 year-olds'. That isn't true. It's a false assumption. Many programs DO begin early...but they build a foundation of social skills in lower grades that lead to well-adjusted adults by college (hopefully). Many include friendship skills, respect for others, abstinence education and self-esteem education at age appropriate times. IMHO, programs that start early, and are comprehensive do a better job.
Hope this helps with whatever you are working on. :)