Because Tim has leveraged Khan’s Course Mastery feature to drive incredible results with his 6th grade students. Specifically, he uses Khan Academy to take them all the way back to kindergarten (yes, kindergarten!) to fill in gaps in their foundational skills – and then to accelerate them to grade level and beyond: search youtube for; How I Helped My 6th Graders Ace Math… By Taking Them Back to Kindergarten! | Mastery Learning
*Just as reading is a fundamental for Science, History, etc.. The basics of Math are essential to build upon. www.khanacademy.org is free and is an excellent source for math and other subjects. You can test your students on Khan in each level of Math to see if they are missing essential building blocks.