If your teen doesn't want to attend school, you need to assess the nature of his refusal and the reason for it. Specifically, truant teens require a different approach than teens who try to avoid going to school, which is different again from teens with a clinical school phobia or social anxiety disorder related to attending school. Once you know the nature of your teen's refusal, you then want to clarify the reason(s) for it, which can include boredom, bullying, an embarrassing event, lack of sleep, wish to avoid tests or specific teachers or rebelling against authority. Understanding both the nature of and reason for your teen's refusal to go to school will then help you find the right solution.
You can classify your teen as truant if he doesn't involve you in his decision not to attend school. Meaning, your teen will leave the house each morning, but just not make it to school. Your teen will likely try to hide his lack of attendance from you. Common reasons for truancy are boredom or challenging authority. If your teen is merely avoiding school, this will be apparent because it's probably a response to a specific event: Your teen has previously gone to school without issue but now is suddenly working hard to avoid it. Your teen will try to convince you, either through imaginary illness, not getting up in the morning or flat-out whining, to excuse him from school. In these situations, you might want to explore if your teen is suddenly getting bullied, is trying to avoid specific school activities or had an embarrassing event.
The most serious reason your teen might not want to attend school is if he is suffering from a genuine phobia or stress disorder at the thought of attending school. If this is the case, your teen is likely suffering from some physical manifestations, as well as emotional responses, to the thought of attending school, such as sweats, shallow breathing, increased heart rate or nausea. However, you'll need professional help to determine if your teen is just a skilled avoider or truly suffering from a stress disorder.