I agree with kermi...by the age of 12-13, many kids' bodies are physically ready, but they are not emotionally ready.
On another note, I also know that all of these stories are true. It is very common to hear stories where I love about so-and-so doing something with so-and-so. I heard these stories as early as my 8th grade year. All the stories about people having oral in the classrooms, etc., are true. Although it has never happened in any of my classes (to my knowledge), I hear about it all the time. One of my own friends had oral with 2 girls in our school's auditorium during school hours. I was shocked when I he told me what he had done.
Largely, many of these people are just looking for quick pleasure. Some get what they want and think nothing of it, but also many are left emotionally devestated after what happens. I know a couple of my friends who had sex at an early age, and after they did it that one time, they realized that it was too early and what could have happened, and they aren't doing it anymore.
Whatever the case...12-13 is just way too young...