I have found the following websites to be great resources for challenging Common Core math and reading problems:
- http://www.readworks.org/ (setting up an account is free!)
- https://www.illustrativemathematics.org/ (these are more like lessons you can teach your student, or have them watch one of the videos)
- https://www.khanacademy.org/ (watch videos of lessons and work on problems from 3rd grade standards, or learn new 4th grade standards)
- http://www.commoncoresheets.com/ (good for math, but also has great reading/writing!)
- https://www.superteacherworksheets.com/ (some worksheets are free)
- http://www.education.com/ (free up to 5 downloads a month)