Judging from, for the most part , narrow responses above, the US educational system has been underfunded

You might think so, but when international comparisons are made on a per pupil expenditure basis, the US ranks very high. The problem is that we just don't get good results for the money.

The reason why is complex and does not lend itself to a simple answer. It is mostly a number of small things that tend to add up.

For example, sports is a bigger deal in US school systems than overseas, and a major drain on school system budgets. School childen in other countries may be equally engaged in sports like soccer(association football), but the activities tend more often to be organized outside of the umbrella of the school system.

Another example: In many other countries, there is a willingness to "track" children when they are still quite young, on the basis of academic performance and especially test scores. Those in the lower tier are shunted early into a vocational track, which sometimes ends up as an apprenticeship program at an employer (and thus possibly not as a school system expenditure at all). In contrast, in the US we are reluctant to track children, and want to keep them all together in the same classes and learning the same things for as long as possible. This is based more on ideology than on any real pedagogical rationale. We try to provide a lot of vocational training within a lot of general purpose secondary schools, rather than concentrating it in separate specialized facilities to which vocational students are sent. This is a very expensive and mostly ineffective way to do vocational training, but it keeps all the students together in the same school. Thus, our students go all the way through twelve grades and are still not prepared for skilled employment; they usually have to go on for an additional year or two at a community college in order to get sufficient training to be employed for anything more skilled than flipping hamburgers.

These are just two examples. By themselves, they are no way close to being a full explanation. But they are only examples, there are many other things like this, and taken together, they do add up.