Is it state law in Missouri for the public school to bring in a medical clinic to teach 7th graders about abstinence??

Like most states, I believe yours allows the schools to develop age-appropriate content for sex education - NOT to leave it to the parents. Unfortunately, you pay the government to make the decisions, you don't hire the teachers directly. While they work for the government, and the government works for you, there are too many opinions held by too many different taxpayers to make them abide by each of your preferences. By law in your state, "teen dating violence prevention" including all tools needed to deal with that is authorized from grade 7 up. Content must be factually accurate for all sex education, age appropriate, and based on "peer review" - that is, simply put, in the mainstream of the majority of the people in the field from which the content is developed. I'm not a lawyer in your state, and this is general advice only. But there are statutes in your state which guide school districts in this. You are certainly free to express your objections to the principal and superintendent, but they are not obligated to remove curriculum even if the majority of the people in the district don't want it - no more than legal protections for the accused would be removed if the majority wanted them to, or that evolution be not taught if the majority wanted it to where peer review was a standard.

To follow on what William said, then your choice would be to remove your kids from the current school and enroll them in a school (most likely a private school) that shares your beliefs. This situation is more a political question than a legal question.