Keep her pre-occupied, like telling her to heal while walking etc.. I imagine if she is in agility classes at that young of an age she must of been in obedience before.

Also giving a quiet commanb like lorraino said will work.

She is at that age where she may become a little rebelious, testing her limits. But remember all dogs bark, especially being a heeler, like any other hearding dog, they bark a lot! It's just part of their breed.