Some people will want to have multiple partners. This can increase the risk of exposure to STDs. BUT if it is with a group of people where the rule is not to have sex outside of the group without consent from the group (preferably with a rule that the person get tested before joining the group) then that can also prevent STDs. People should hear about this practice so if they really are intent on having multiple partners they can protect themselves.
I really don't think the average high school student is worried about have multiple lovers at this point in their lives. I think they view them as stories and not that of truth. I know I did at that age. I went to high school in the late 90's It has changed,but it still not Sodom and Gomorrah.
I think discussing polyamory is appropriate in high school as long as it is done in a balanced and objective way. Students need information in order to deal with situations that may happen to them in the future, and restricting information does not prevent them from experiencing undesirable things, it just reduces their ability to deal with it sensibly.
To me, this question is tough. I think of my son being in high school and coming home to ask questions about polyamory and I don't like the thought, but in the end I suppose it would be appropriate, especially in a high school health class. I may not want my child to grow up, but he is, and I want him to be aware of the world around him.
Unless the plan is to tell them not to do it, there is no reason to have polyamory stories in a high school health class. The public schools or private ones for that matter do not need to be telling children that risky behavior is normal, popular, or something that they should be doing.
High school health classes should emphasize safe practices and educate students on care and caution. They do not need to become classes of acceptance for any sexual lifestyle as that is a preference not under the domain of education. If parents feel such stories are appropriate, they should be the ones to determine whether or not they are learned.