In our schools there are two tracks, the 'gifted' and the other. The gifted track graduates will go on to college, sometimes to very competitive colleges. The standard track graduates are more variable. Some will become teachers and entrepreneurs, others will compete for the shrinking pool of blue collar jobs, and some will struggle in a world that has less and less need of the non-elite.
Special needs students are mainstreamed within those tracks. High IQ autism-spectrum kids may be in the gifted track and may graduate with honors. Low IQ special needs will be in the standard track but will not receive a high school diploma . We have one child in each category.
The standard track is obviously challenging for teachers. Among his or her students will be some gifted kids with quirky temperaments, many average kids, some kids with environmental issues, and the bottom 5th percentile. Some will be labeled "special needs", but in terms of performance and behavior those kids may overlap with the bottom 10 percent. If a special needs child doesn't have obvious physical signs of disability the overlap will be particularly difficult for the teacher.
So how does a teacher grade those students?
If she grades 'fairly' the special needs kids will get a zero on every assignment. So will the low IQ kids who haven't been labeled. If she adjusts grades according to effort there will be a lot of painful judgment calls and complaints. If she grades the special needs kids on their own curve they'll do far better than non-labeled low achievers, which demoralizes them.
In practice the better teachers divide kids into diploma and non-diploma candidates. They grade the non-diploma candidates roughly on effort, and the low achievers on a curve. Their reward for this work is to get more work.
There's an easy out for the teacher who wants less work though. Simply grade everyone fairly. Give the low IQ special needs students the same grade they'll get on Minnesota's special-needs-soul-crushing standardized tests - zero. After a little bit the special needs kids will give up. Their parents will campaign for a different teacher, or the child will get labeled 'EBD' and removed. It's a temptation some teachers can't resist, especially if they feel unfairly treated themselves.