You really can't limit nicknames that come from other sources. You and your husband can always call her Juliet, but if her school friends call her Jules, they're allowed to do so if she's okay with it. That said, I do think Jet is a cute alternative. Lettie is also a nice possibility.
Middle name suggestions:
I have a ten year old daughter named Juliet and almost no one at her school or out in the world calls her a nn at all. It seems to be no problem. It is a great name, imo! She has never had another one in her class, yet everyone knows of the name, can say it, etc...Most people think it's pretty. My Juliet goes by Juliet at home mostly, but also Julietta, Jules, and Jetta. Her middle names are Juliet Karenna Lorraine.
Other middles I like are:
I seriously disagree with previous posters about nicknames. My son is Bennett and we were determined that he would NOT be Ben. So, we just correct people nicely if he is called that. He would as well now that he is older. When he is a teenager, then he can do it if he wants. If people call her Julie or Jules, you just say "Oh, it's Juliet". That politely takes care of it.
Clara and Juliet are two of my favorite names!