substitute out another class
Many consider this the ideal. Rather than coasting through a class in which a student is "treading water" while waiting for others to catch up and/or complete their work, the student would spend time in school learning new information: being presented with classes and materials at the appropriate level to provide challenge, learning, and growth in his/her zone of proximal development (ZPD). Many families and schools partner on planning and providing single subject acceleration, such as math one or more grade levels advanced.
Taking the end-of-year test early is one way to prove a child needs a more advanced curriculum placement and/or compacted pacing. Results of above-level tests may also be useful as evidence that the student needs or would benefit from higher curriculum.
On another recent thread, there's a post which lists common downsides to NOT providing higher level curriculum for students with a demonstrated need for it:
negatives, such as:
- lack of challenge leading to boredom,
- loss of sense of discovery, exploration, curiosity,
- development of an unwillingness to struggle and risk making mistakes,
- dearth of opportunities to learn from one's mistakes/failures,
- failure to develop perseverance and resilience.