When I was still working in Poland, about 6 years ago, I had a student who acted quite troublesome in class - he didn't follow instructions, he was turning his back to the board, speaking over; basically, he was quite disruptive. That was an avoidance measure. He was simply unable to follow the proceedings in class, struggling with spelling and writing mostly, but also (now that I think of it) was likely APD as he seemed to be unable to easily understand instructions issued by myself.
I referred him to the school psychologist/social worker/language department - and it was determined he was indeed dyslexic. Once that became clear, I sought guidance from the social/psychology/language department on how to approach this matter and soon after I was running one-to-one classes with the student, tailoring classes to him. The school didn't employ any assistants or ASN teachers at the time, but the student still benefited from the additional classes - and the challenging behaviour coping mechanism whilst still present was diminished very significantly.