Yes a student with ADHD or other disabilities can be expelled from school just like any other regular students can.
However most schools will not expel students if they have ADHD as quickly as they would other students without ADHD or other disabilities.
When a school expels a student with ADHD or other disabilities they put the school at risk of lawsuits and litigation.
That's especially true if the expel reason was because of the behavior associated with the ADHD or other disabilities was what caused the child to act up and get out of control.
When a student with ADHD or another disability is expelled for their behavior because of the disability or ADHD the school would be putting their LEAs and SEAs at risk of litigation for failure to provide an appropriate public education.