In a nutshell, the Core Knowledge Sequence provides schools with a clear outline of what should be covered in grades K-8 so that all students are gaining the essential, basic framework of common knowledge. (For example, students should not have gaping holes in their knowledge of American History because they studied rain forests for several years in a row in grade school.) A Core Knowledge school can supplement and add in other areas of study or topics to suit the needs of their mission and population.
(James Irwin Charter Middle School is also a Core Knowledge school. The high school continues to build on the foundation of learning by offering a challenging college preparatory, liberal arts curriculum.)
You can learn a lot by looking at a general overview of the Core Knowledge at
http://www.coreknowledge.org/CKproto2/about/index.htm
You may also find more detailed information at the Core Knowledge link for Frequently Asked Questions, http://www.coreknowledge.org/CKproto2/about/FAQ/FAQ.htm