How to address a Hiring manager whom I know personally

Hello Job Forum Community,

I’m applying for a job in the school my son attends. I have been a founding member of this school, worked in the PTA and was a member of the Board of Management for a while. I know everyone who could possibly view my application quite well. There is a very relaxed atmosphere in this school and everone calls each other by the first name.
Of course, I want to write my application in a professional style, but Sir and last names sound akward when I have never addressed them like this before. Is it a no-go to use first names in a cover letter if the rest is written in formal style?

Thank you very much.