Think about every one of your teachers for a moment. <think>
Done? Now, roughly what percentage of them would you want telling you about life (when you're brain is not physiologically mature enough and you aren't old enough to have life experience to think rationally about what all of these teachers say)?
What do you think the correlation is between teachers who want to convince all the kids to see things exactly the way they do, and the teachers that are likely to speak up? Probably very high.
If you don't want a Christian teacher teaching your kid about how there's a heaven and you can only get into it if you accept Jesus Christ as your personal saviour, you can apply the same reasoning to why lots of people don't want teachers teaching their kids about complex adult relationships.
Parenting is for parents; teachers teach.