J.F.
I think your approach is good. However, you need to realize that they hire teachers for the fall in March and April. You've somewhat missed your window of opporunity. IF there were any positions open (that they knew of), the principal would have interviewed the MANY candidates in the spring. As the year ended, they would have known almost all of the open positions, and continued to hire for the fall from the pool of spring candidates. Then as the fall approached the principal may have had 1 surprise opening left and filled it with any of those candidates.
If I were in your shoes really needing a position, I would put together a resume folder that included your resume, cover letter, and even your Philosphy of Education. I would include in my cover letter that I have a portfolio that I would be able to show at an interview. I would leave the folder in an envelope (resume type) with the secretary addressed to the principal. If he/she responds in any way, send a thank you card for their time. Then, don't be pesty.
In the mean time, you need to get as many sub positions as possible and aim for one that is a long term one (maternity leave). These are the people who will get the most notice, especially if they can prove their stuff.
With all of the cuts in education there are many teachers who work for a year and then are laid off. It is not an easy market. As well, many of the older teachers have retired already. There are less and less openings.
Good luck with your search. Decide how important it is to get into the classroom, and work at doing it. :)