Leaving aside the issue of contract for the moment, an employer has the right to tell employees they may not have vertain outside jobs or, indeed, any outside jobs. That flows from the doctrine of "employment at will," which is the law of the land except and only to the extent limited by contract; among other things, this means that an employer may set any terms or conditions on employment, including no outside employment or limitations on outside employment, that it wishes.If you do have a contract, review it. If it only allows you to be disciplined or terminated for certain reasons and coaching elsewhere is not one of them, they can't take action against you for doing that; but if there is any room under the conteact to take action against you, they may.