How could teachers help anxious students?

Hope this is the right place for this question. I'm a high school teacher and I wonder, if you have anxiety at school (especially for tests and marks, but also in general for being at school), what would you like teachers to do that could help you feel better?
I have a student in particular that has this kind of problems, and I try to be there for him, but some more inputs wouldn't hurt.
(I want to add that I'm a support teacher, so I can't decide how to lead the class activities or how to test him...this maybe limits a bit what I can do. But the positive thing in my role is that sometimes students trust you a bit more; for example, when a couple times I saw this student of mine upset for something or down because of a low mark, and I tried to talk to him, he did open up a bit).
I don't want to be pushy and I know I can't do miracles, but since he seems willing to get the help, I'd like to do and/or say useful things if the occasion comes.
Thanks if you have ideas

You can do diaphragmatic breathing which is a type of deep breathing. Very effective. Person is able to access it all the time. I think it is a useful skill and easy to learn.

I asked my daughter, who's in high school, this question.

These are largely her thoughts, and she's quite good at managing anxiety as a general rule.

She feels that anxiety in people her age is often the result of not having the tools and resources to assess and manage the source of the anxiety. That's not always easy to pin down, but she says she feels like a lot of her peers (and sometimes herself) can't easily get a bead on what it is they're actually feeling, so it comes out as generalized anxiety. Are they scared of failing this particular test? Do they not feel prepared? Maybe they don't know. A lot of adults don't know.

For her, she says someone who will just listen, and maybe provide a small amount of a sounding board, is helpful. Though, keep in mind, that it can take time for a kid to become comfortable and open up. Adults can mirror back, and maybe even expand on what a kid may be feeling, which can help them process what's going on. But just listening and understanding can go a long way to helping a young person learn to work through their feelings. This may be especially true with generalized anxiety.

Of course, letting them know that anxiety can be normal, and helping them learn to calm themselves in the moment, is also good. Along the lines of the breathing exercise mentioned above, from the time DD was in elementary school, I taught her that if she started to feel overwhelmed, to stop whatever she was doing, place her hands flat on the desk, and just breathe for about a minute. It can be a good reset when your head starts running faster than the rest of you can keep up.

Another thing DD mentioned was that it's easy to feel overwhelmed in high school - there's often just way too much going on at any given time. Some kids could use a step back, and a little help organizing a realistic way to proceed. The example she gave was having way too many assignments and obligations and feeling so overwhelmed that you just shut down. If you can figure out the kid is overwhelmed, helping them form a bit of an action plan could be helpful. I.e. help the kid sort out what's important ftom what's not, and then begin to prioritize the things that are overwhelming them. Then they can work through the list in a methodical way that gives them a sense of control.

Don't know if that's helpful to your situation, but that's more or less what I got from the resident HS student.