Highly intelligent people struggled in school, like Albert Einstein. He was deemed a failure, but his mother never gave up.
Find out why she's not engaging. Sometimes really bright children find doing the mundane worksheet of 30 problems a bore. Yes, she could probably whip it out in a few minutes, but it may not be stimulating enough. Compromise.
As a teacher, I do a lot of that. When Little Joey can show me he can do 5 or 6 of the problems I select accurately, he is done. Sometimes, Little Joey ends up doing 10 or 12 problems, but he has shown mastery and can move on.
Talk with the teacher about what he/she is seeing in the classroom.
Eliminate any health issues. My 1st grade daughter was not doing her best in school, so I took her in for a physical. She was fine. I had her eyes checked. She was fine. Accidentally, we found out her allergy medicine was making her brain feel "sleepy," as she put it. A yr ago, we added a new medication for an unknown allergy that was causing skin and respiratory issues. When we took her off all meds at Christmas to have a full assessment done, we found we had a vibrant, lively, creative, chatter box again. (We thought she was going through a quiet, introspective stage) Then it clicked. She didn't get tested and isn't on any meds. I have a thinker and problem solver now who is engaged at school and doing well. My 4 yr old reader went underground, but came out again at 6 as a chapter book reader.
Make sure she's healthy on all fronts and talk to the teacher.
When I make arrangements as I did with Little Joey, I have parents and children present to make the agreements all will live by at school and at home.