Loosely, it's any student with an Individual Education Plan. An IEP requires the teacher and principal to meet with a parent at least once a year, by typically it's several times per year, first to establish a plan for meeting the student's needs, then to discuss the student's progress, or lack thereof. Additional staff might be in those meetings, such as math or reading intervention instructors, school psychologist, etc.

It's possible for a student with a learning disability to not be disruptive in class, and for a non-IEP student to be disruptive in class. However, generally speaking, IEP students represent the majority of the discipline cases.

And, a large percentage of IEP students does mean a school district will be spending more money meeting the needs of its worst performing students.

There's a saying among teachers that this group of students 'have all the rights' because of legally mandated funding for them and how schools are organized around meeting their needs, rather than the needs of mainstream students.

Meanwhile, on the other end, advanced students are under-served and the state allows school districts to redirect GATE funding to non-GATE uses (GATE is gifted/talented education).

My school district was reorganized last year around high need students, with a language immersion program (students taught a second language beginning in kindergarten) being gutted because high need students not in the immersion program couldn't be mainstreamed. They turned two K-5 schools into a K-2 and 3-5 schools and gutted the language program so the two populations would be in the same classroom. And thus, more middle class families fled.