In high school, one sign that you teen is not being challenged is skipping class! In grade 10 my son skipped one of his classes a few times. I got a call from his teacher and I immediately called a meeting with the teacher, the VP and my son.
After my son was confronted and we had a really great discussion at our meeting about his education and his future, we learned that he simply wasn't being challenged and therefore was not interested in participating in that class. From that day forward, they changed things up a bit for him to ensure that he could successfully earn the credit. He completed grade 10 with an 80% average, he completed grade 11 with a 90% average and he is well on his way to graduating next year! It turned out that all he really needed was to know he could communicate with the school staff and they would work with him.
Sometimes teenagers will not use their voices... they act instead. That is when it is very important that we as parents pay attention to the red flags, get involved and find out what the issues are instead of just getting angry or simply ignoring the problem. He hated that I got that involved at first, but he thanked me later! ;)