The rule says "black ribbon, tied in a bow, may be worn". You can no longer use a length of fabric tied in a knot. Try grosgrain ribbon about 1" wide, tied in a bow and pinned to the top. It doesn't actually have to go around the collar and neck.

You can't get grosgrain ribbon 1" wide - it's either a little less or a little more than 1".

Anyways, we have had great luck folding the end of the ribbon and tacking it at the shoulder seam, under the collar. It doesn't get lost, and it sits better when tied.

...and to add to the last two posts, I find it is helpful to pin the centre of the bow together to prevent it from coming untied, which can sometimes be a problem.

Thanks! I feel like running off the the fabric store right now!