There are more opportunities for education in America than in Mexico. While there are some very good schools in Mexico, there is not universal access to education. Many rural areas have no schools, and areas with schools often require uniforms or tuition. There are fewer schools, and fewer universities.
I think you need to be very careful when trying to compare and contrast educational systems and results between a country that is classified as 'developing' and 'developed' countries. One of the differences is the accuracy and amount of statistics that are kept in developed nations compared to underdeveloped nations. As highlighted above, the need for many children to work outweighs the need for education, and so you can't really compare the education system of these two countries fairly.
Educational attainment in Mexico is much more difficult than in the United States. One of the major problems that the Mexican education system is low enrollment. This is mainly due to the fact that poor families, especially in rural areas, need their children to work, and they don't have time to attend school. In addition there are simply not enough secondary schools (high schools and colleges) to meet the number of students that could be eligible to attend.
A second major problem is economics. Mexico is a poor developing country that does not have the money to spend on things like better buildings and facilities, supplies, teacher training, etc. Without these things student learning suffers. A good education costs money.
In addition there is little educational research in Mexico and very little transparency in its government agencies running the schools. It is hard to know what is being done right and what needs improvement when there are few statistics and little opportunity to investigate.
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I would argue that the most important difference in educational attainment between the United States and Mexico is the difference in the literacy rates of the two countries. Mexico's literacy rate has been improving, but it still lags behind that of the United States. In Mexico, the literacy rate (defined as the percentage of people over 15 who can read and write) is 91%. A little under 90% of Mexican women of that age are literate. By contrast, the United States has a literacy rate of 99%. Importantly, the rate is the same for women as it is for men.