The speed of light or the speeds of light?

The Italian government’s Ministry of Public Education, which defines the high school curricula, has introduced almost all the topics of modern physics for students in their last year of high school. The theory of relativity is, perhaps, the argument most developed. The students, doing a web search, read that the speed of light can take different values in General Relativity (GR). We have noticed that this concept creates a lot of confusion and also a misconception in young students. In this paper, we want to describe a lecture given in the classroom about this topic.