They're both brutal bloody films centred on a group of high school students' massacre. While the overview is similar, the plots are quite different since Battle Royale is about a surviving game where students kill each other while Lesson of the Evil is about a psychopath killer teacher.

In Nagareboshi, the male lead marries the female lead so that she will be a donor for his sister while in Platonic, the female lead marries the male lead so that he will be a donor for her daughter.

Everything else is quite different since Platonic takes a darker tone while Nagareboshi is lighter in comparison.