-Lack of proper implementation of common core. Veteran teachers weren't trained for the common core. The standards are also controversial as they were developed by people who aren't experts of education or developmental psychology or anything like that.
-High stakes testing - leaves little time for authentic learning and forces teachers to "teach to the test." Children's social and emotional development also get ignored. Which isn't beneficial toward their academic success.
-Lack of funding. The kids who get neat manipulatives for math, go to field trips, and have new technology that allows the teacher to have more time for planning, etc. are generally more motivated and successful kids.
-Ineffective teachers - for a few reasons. Some are burned out. Some don't promote authentic learning; they're centered on lecture and rote memorization, rather than hands-on activities or visuals. Some aren't anywhere near experts on the psychology of kids or young adults - whoever they teach. Some don't know enough about the psychology of learning and memory, which ties almost directly into authentic learning.
Luckily, there are a lot of amazing teachers who diminish the effects of factors contributing to poor results in school. Go amazing teachers! =)