With regard to educational oppression: to what extent has the lower socioeconomic population been denied access to the educational system or given unequal treatment while in the system (current affirmative action issues relate here as well)?
The answer to this varies with the part of the educational system that we are referring to. There is generally very little exclusion of the lower socioeconomic population in the elementary and secondary school systems. The real exclusion comes at the college level. At the lower levels, there is unequal treatment rather than exclusion.
In the United States, the main type of unequal treatment is inequality between schools in richer and poorer neighborhoods and communities. In the richer communities, there is ample tax funding to support the schools. It is also generally much easier to raise large amounts of money through fund-raisers. Because the richer schools have so much more money, they are able to provide more kinds of programs than poorer schools can. This disparity is exacerbated by the fact that the best teachers are more likely to go to the better schools where they are likely to have fewer frustrations with low-quality students and facilities.
There is, however, some degree of unequal treatment even within schools. It has been documented, for example, that children of color (who are more likely to be in the lower socioeconomic population) are more likely to be suspended from school and are more likely to be placed in special education programs. This implies that administrators are not treating children of color fairly when compared to white children.
The real exclusion in our system comes at the college level. Poorer students are generally excluded from the elite universities and have a much harder time getting any higher education. This is partly because they tend not to be very well prepared for college. It is also because our system is not very good at providing aid to students that will allow them to go to college.
Thus, we can see that there is a great deal of unequal treatment, as well as some exclusion, of lower socioeconomic populations in our school system.
Paulo Freire is an educator in Brazil, who would be someone relevant for you to look up, as he is primarily concerned with oppressed education.
One of the most obvious examples of the lower socioeconomic population being denied a proper education is the materials they use to learn. In some neighborhoods, students do not have enough textbooks to be able to take materials home and study, but other, more highly funded schools, are able to give electronics like iPads to every student to keep for the entire year.
Schools in richer neighborhoods and districts are at the point where they have so much funding that they are building more public schools for their students, when other districts must combine classrooms and teachers. In the United States as a whole, states lie California are struggling for school funding, but states like Texas have such a plethora of funds that they are able to give their students more. But this problem goes deeper than just the students. Teachers are being layer off and finding it harder to find jobs because schools do not have enough money to pay their salaries.
On the whole, there is an incredibly unequal distribution of wealth in the United States's public education system, which gives children from poorer economies less of an education.