For my son he took the beginning of the year test (which is the same as the end of the year test only different numbers) for second grade and got a really high score (like 80%). So for the next two weeks the teacher gave him end of chapter tests one after the other (really just to have physical proof that he didn't need 2nd grade math). When he got to a chapter where he wasn't getting 100% on the chapter tests he stopped and worked through the chapter. Once all the chapter tests were done he took the end of the year test. She then walked to the third grade work room and picked up one 3rd grade book, workbook, the book for extra challenge, and the testing book.
Her strategy was...each math period the rest of the class got a set of 5 warm up/review problems projected onto the smart board to work on individually or in pairs. During that work time she presented my son with his math lesson. Then he got to work while she presented the class their lesson. Then she walked around the classroom helping kids in their independent work, during that time he could get additional help or get a challenge assignment if he was done. He also had access to tons of math programing on the computer (his math series has an online component and they had several additional websites they subscribed to). So that went on all year plus he started in on a bit of 4th grade math.
As far as I could tell it wasn't the least bit disruptive to anyone's learning and just woven into how she ran the class.
But when I was a kid that is how classes were run. Usually two groups (sometimes 3) different reading groups in one class and the teacher divided her time. There were two teachers per grade in my school and usually one teacher taught math to a whole class but the other teacher sometimes had two math groups and sometimes an individual kid walked up a grade to that math class.
