The Kabbalah is a mystical text and, in Judaism, is not meant to be read by anyone under 40 years of age. Considering that the text focuses on mystical beliefs and is not reflective of Judaism as a whole, it seems a though including it would be both disrespectful of the religion and misrepresentative of it.
Kabbalah is a religious teaching among Jewish traditions and beliefs. So schools in Israel and even schools with large Jewish communities surrounding any area are free to teach it to students who are interested. But to mandate this teaching to every school is unnecessary and completely unethical to those who don't really have any significance to it.
I personally think that Kabbalah being taught as a part of religious courses in high school could affect some people and what they might believe in. I personally think that Kabbalah being taught as a part of religious courses in high school could affect people's religion as well as the course in which they are taking.
If it is a world religions class in high school, then of course Judaism should be taught along with other faiths. But every tradition has its own mysical tradition that is more difficult and esoteric. The old thinking about Kabbalah was that everyone who studies it needs to be middle aged to get the true meaning.
Given that Kabbalah is a sect of a larger religion, it is best that it not be taught as part of religious courses in high school. While an overview of religions and their benefit to history is appropriate to teach to high school students, religion should not be delved into with any great detail in non-secular schools. The major tenets of large religions should be the main focus of the courses.