Well, that answer has many possible answers. It may be the case that the teacher was also taught the incorrect information and unwittingly passed the misconception on to his/her students. It could be that he/she was aware of the primary colors being cyan, magenta, and yellow-- and just wanted to simplify vocabulary for students by calling cyan "blue" and magenta "red". This oversimplification is a problem, though. While it makes sense in everyday language (i.e., there are lots of shade of "blue"), to a scientist "blue" and "cyan" have very different meanings.
So why cyan, magenta, and yellow? Well, here we get into some physics. The primary colors of light, or the "additive primaries" are red, green, and blue. If you look at a projector or television screen, or scrutinize your white computer screen, you can see these three colors. Objects appear as "colored" to us based on the light they absorb and reflect. Objects that absorb red light reflect blue and green light, and thus appear "cyan". Objects that absorb blue light reflect green and red light, and thus appear "yellow". Objects that absorb yellow light reflect blue and red light, and thus appear "magenta". Therefore we have the "additive" primaries of cyan, magenta, and yellow. If you look at a color printer cartridge, you will notice three dot- cyan, magenta, and yellow. These are the colors of ink your printer uses to make ALL other colors.
Unfortunately, we are rarely able to find pigments that are pure cyan or magenta, so artists have reverted to using red, blue, and yellow. As you have probably seen for yourself, these three colors work well to make just about any color as well. Thus, the "primary colors" of red, blue, and yellow grew out of color mixing preference, not color science.
You can find out more information about this phenomenon by visiting the following: