A five year grappler in the beginner division would be the definition of sandbagging. Don't let your coach talk you into it.
Most competitions say "Beginners = less than one year" or something like that. I have never been in a tournament where a five year guy wouldn't be cheating by going beginner. If this tournament says, "Experience level as determined by instructor" or something weird like that, then it's a different scenario. But, I'm guessing this tournament has a certain amount of mat time they are expecting you to be under if you go beginner. If you have more than that, then you are cheating no matter what your instructor says.
Most competitions say "Beginners = less than one year" or something like that. I have never been in a tournament where a five year guy wouldn't be cheating by going beginner. If this tournament says, "Experience level as determined by instructor" or something weird like that, then it's a different scenario. But, I'm guessing this tournament has a certain amount of mat time they are expecting you to be under if you go beginner. If you have more than that, then you are cheating no matter what your instructor says.