One of the biggest controversies about sexual health education (SHE) at the K-12 level is just that - the fact that it is taught in school. Some claim it's inappropriate - this is not the sort of subject that should be taught in schools. Others think it's up to parents to teach their children SHE. The list goes on.
I'm going to focus on the two aforementioned arguments against SHE at the K-12 level (even though there are plenty of others). Firstly, I personally think that offering SHE at school is one of the most relevant places to offer it. When you go through teachers college or any formal training to become a teacher, we often discuss the concept of the "Whole Student". As teachers, we have the moral obligation to ensure that every aspect of our students is nurtured and supported - this includes their physical, mental, psychological, emotional, social and academic well-being. Sexuality impacts each aspect of students' identity. Physically: one word - puberty. Mentally/psychologically/emotionally: it is very normal for children, as early as pre-school age, to have questions about their sexuality. Kids are curious and develop at different stages, and sexual development has (positive and negative) influences on one's mental, psychological, and emotional states. Socially: A student who identifies as trans, for example, may be faced with transphobia and as a result, would likely experience challenging social conflicts with their peers. Academically: Continuing from the previous example, if the same student is bullied during their transition, their academics will likely suffer because of their emotional distress (related to my previous point) from having a challenging social life, which is a very important part of childhood/adolescence (though it is important to note that there are many other factors that contribute to emotional distress during transitioning). My point here is that sexuality is part of an enormous, complex web that we call Identity, and we cannot selectively choose to ignore one part of our students' identity when it has such a powerful impact on other aspects of their identity and life in general - both inside and outside the classroom.
With regards to parents teaching SHE, Planned Parenthood Toronto - in partnership with other organizations - conducted the Toronto Teen Survey in 2010 (see link/document attached below). This was one of their findings:
That's right, youth (participants were 13-18 years old) were least likely to ask their parents (28%) about sex-related questions. They were more most likely to ask their siblings, call an information line or speak to a semi-professional (55%), followed by friends (53%), professionals (such as doctors and nurses, 43%), and the media (33%). This is not to say that parents should not have any conversations about sexual health with their kids - in fact, SHE is much more effective if parents continue the conversation at home with their kids. Many of the SHE teachers I know often send an email home before their SHE unit outlining the topics they will be covering and encourage families to talk about those subjects before, during and after the unit.
In Ontario, SHE is part of Health and Physical Education. The purpose of Health Education is to help students make healthy choices. It's about giving youth the autonomy and agency to make their own educated and well-informed decisions. We cannot stop development, we cannot ignore questions and hope they go away. Teachers have a responsibility to fulfill, to make sure that their students are healthy in every part of their lives. Sexual health included.