This site provides games at different difficulty levels for each subject matter. For both math and spelling, there are games at “Easy”, “Normal”, and “Hard” difficulty levels. These levels refer to
the ease at which a student can play the game, not necessarily the questions themselves. Each level has at least three different games for a student to play. I enjoy this feature because students who are struggling can choose easier games while my high flyers can challenge themselves. It also allows students to have choice, while switching things up to hold engagement.
Their math curriculum has been mapped directly to the math CCSS from kindergarten to fifth grade. On their “Math Curriculum” page you can see a detailed description of how they adhere to the standards.
Inventory (TPRI) and the Iowa Test of Basic Skills (ITBS). If you are curious about the types of questions students encounter while playing these games, or about the validity of their curriculum, they have their entire curriculum mapped out for anyone to see on their Math and Spelling Curriculum pages.