This is a hard lesson to learn for any stage manager, either in High School, College or in the professional world. (There will always be someone who has a "higher status" then you, even on Broadway, there is going to be those pesky stars). Now is a great time to start developing your management style. Often, young stage managers are nervous about being the "leader" or "manager" for a large group of people, but at the end of the day that's our job. I find it interesting that you are comfortable with the tech crew, but have an issue with actors. This may be because of the age difference or experience issue.
I think many stage managers use just blunt force "I AM THE STAGE MANAGER, LISTEN TO ME" sort of power trip to be assertive, which is at times can be easy to fall back on, and but can be a danger down the line. The best way to lead a group of people and thus be assertive before them, is to earn their respect and trust. There are two ways of doing this, the easiest way to is wrestle a bear and win in front of the cast which will require some mean bear fighting skills and of course a bear or, the way most SMs do it is leading by example, doing their job well, treating people fairly, be rock solid. Trust me, the wrestling the bear is sometimes easier.
Your question is really a very complex one, and one that you will be constantly working on it for the rest of your career because it is all about your comfort level and management style. Its all about the relationship you form with your cast and crew, and that starts from your first encounter. My management style is based on mutual respect and professionalism. I am always give the respect to the actors and their work, and in return, I expect the respect for the job I have to do. From the get go, all me encounters are somehow based on this in the back in my head weather it be a season actor, a acting intern, a designer, or a crew member.
Now, to give more concrete advice please, dont wrestle a bear. Try finding your comfort zone. Do you have an easier time talking to actors when they are all gathered in a group when they are standing around, or when they are sitting in the house? Are you better to assert yourself if you have notes on the things you need to talk about? Does opening with a joke help your confidence? I would recommend taking acting classes (helps with confidence) or public speaking classes. Also, find a management book that may work with your style. If you are more confident in your management style, then you may find it easier to be assertive. (I recommend
The Heart of Coaching: Using Transformational Coaching to Create a High-Performance Coaching Culture (3rd Edition) by Thomas G. Crane and Lerissa Nancy Patrick (Paperback - Mar 1, 2007))
Trust me, over time and with experience this will get easier.