Help. I need advice...

Hi, sorry to keep bothering you. We have gotten to the bottom of one of my daughter's behaviour (leaving class).

She keeps leaving class for many reasons (some of which we are still trying to identify) but today was because there's a boy in her class who is extremely loud. She has 3 other students in her class (usually) which are all on the spectrum. She's in a PRU (Pupil Referral Unit) so some behaviour is challenging. Anyways, today (and many other times) she feels the need to leave the class because he becomes very loud. My daughter struggles to learn when he's being a distraction or being loud since she leaves the room. Teachers are aware of the problem and keep telling him to be quiet but he doesn't. She really wants to learn but just really struggles to stay in the classroom, even to the point where she'll refuse to be in the same room with him. 

I just need ideas on how to help her in school, since I'm not there with her. She doesn't care what he does or his behaviour. It's the noise which is driving her (and the other students) crazy. 

How can we help her stay in the classroom? I'm sorry but this is stressing my daughter out, and like every parent, I just want to help her. 

Thanks x