Note: I'm not an academic, but I had a student like this as a colleague while in high school (12-18 in a school for students with Asperger's)* myself and I think that this is a worthwhile subject to discuss.
Sometimes you have one or two REALLY shy students in a class with a small headcount. Students that never raise their hands in class, never ask questions, never notify that they have a problem,... The really severe cases will internally panic when you ask them a question and just shut down completely, not even making a sound or a response.
They aren't bad students, far from it. They make their assignments, score well on tests and succeed in the exercises. They just are REALLY passive in the classroom, to the point of almost starting to cry when the teacher asks them something. They also only have that problem in class: when they're among friends during non-class time, like lunch or break, they're quite open and talk rather freely.
As an example, the student from above was so shy that he couldn't introduce himself during the very first week. In the first 2 years, he couldn't give any public speeches, even after much encouragement from the teacher. You couldn't get any more than 2 consecutive words out of him. When the teacher addressed him because they wanted to involve him, he did the above mentioned shutdown, even after repeated encouragement. He loosened up somewhat over the years, but it was still such a surprise when he first willingly raised his hand (sometime in the 4th year), that he got a positive note in his weekly report just for that.
The problem is that it's not always immediately obvious if the student struggles with something. Sometimes, a student like this hopelessly fails an entire class because he didn't understand something from the first year and was too shy to ask for help.
What methods can a teacher use for students like this?
*While I mention a student from SEN education, this question also can apply to non-SEN students. And while this question mentions a high-school student (which I understand is not entirely on-topic in here), it also applies to higher education with smaller groups.