American senior high school might be weak by world standards, but the topics are far above what I did 25 years ago. Consider these mainstream 10th grade "human geography" questions, based on a college freshman textbook that's too expensive to distribute to students:
1-NIR Map pg.54-What continent has the highest overall NIR rate-explain why you think that is?
2-TFR map pg.55-Why do you think North America has a TFR of under 2.1? (give one reason)
3-TFR map pg.55-Why might the TFR in Africa be so high, 4.0 or above for most countries (one reason)?
4-Babies born in sub-Saharan African countries can expect to live only into what age (life expectancy)?
Yeah, Houston, we have a problem. A problem for special needs students, and, I suspect, a problem for the 30-40% of the student body with an IQ under 100. This might explain something about drop out rates for neurotypical children.
Broader societal issues aside, there's a big adaptation challenge here. It's possible to create an adapted text, but it takes work to translate the topics and questions to a sub-80 IQ. Even if there's a parent willing and able to do the work, a teacher has understand how to manage adaptive grading within the framework of an IEP.
I said 'a teacher', because our school district recently shifted all adaptation responsibilities to teachers, while deprecating the role of special ed teachers. Our experience is that teachers aren't doing this.
We don't know why they're not doing the adaptions. Based on experience with medicine  and corporate life, we suspect the root cause is some mixture of lack of training, lack of experience, lack of local leadership, perverse incentives, lack of interest, and lack of time. In some cases teachers seem to think that minimum wage aides can manage the adaptations (surprise, they can't). Sometimes, as parents, we find ourselves training teachers on basic adaptation principals.
We probably can't fix the "perverse incentives" problem -- especially because we don't know what they are. Instead we need to find ways to put pressure on local principals, so they provide the leadership to address the training and incentive side of the problem.
 Almost everything that's true of medicine is also true of education - and vice-versa.