Having sat in on a few interviews for my school I have learned so many things.
First appearance does make a difference but only to a point. As long as you are dressed nicely, etc., that's generally all that really matters.
Voice - depends on the position - but someone who whispers or talks too quietly may get looked-over. Same if someone is too loud or boisterous.
I think nervousness is part of the equation - I hate being interviewed myself - but not the deciding factor. I know many of the teachers or aides we hired were very nervous but still were able to answer the questions and fit our criteria.
Unfortunately you rarely know the criteria and that makes a difference. If we are looking for someone who will be able to take on the gym classes or music as an extra class and someone we interview has those qualifications chances are they will get the job. They may not be the main position so they will not be mentioned in the advertisement because if nobody had those extra qualifications we would then hire someone else. And often someone gets the job because everyone who was conducting the interview felt that individual would best fit into the school, staff and community.
If you treat interviews as learning tools - each time you will come away knowing more tips and techniques to help you with the next one. My sister-in-law just got a job in her field (Registered Dietician). She just moved to the States and in 5 months I would say she had over 20 interviews perhaps even up to 30. Most weeks she went on at least one interview. One hospital interviewed her 3 times. Then last week she had two job offers on the same day. She knew she was qualified and an excellent worker so she just kept at it.
You have put in your time, you finished your degree, did your student-teaching, it might take time but you will get a job. Don't forget that although it seems more hopeless right now, quite often school divisions have last minute openings in the beginning of September due to higher enrollment figures or teachers needing leaves for some reason. Also over the summer some teachers find themselves expecting a baby so short-term positions also open up - they are a great way to get your foot in the door. And substitute teaching is also an option.
Practice questions, think through your answers. I know during the interviews I sat in on, I was always impressed with those who asked for time to think about their answer or who asked for clarification to the question. And use common sense with your answers - one of the questions my principal loved to use was "Is there any type of student you would not like to have in your classroom"? You wouldn't believe how many said they would prefer not having a student with too many special needs or one with very difficult behaviour problems. Of course if we could always have the perfect class, but we know there are challenges and I know my principal was looking for those who were willing to face any challenge.
Remember you are selling yourself - what is the best qualities you have! Think about them and this is what you want to sell those who are interviewing you on.
Good luck to all of you, I really do hope that the right jobs come along for you soon