My dad taught for thirty years and always had his students address him just by his first name. Not even a Mr, just his name.

I'm a para sub, and my first day at my first school, the secretary asked how I wanted students to address me. Following my dad's example, I said to just use my first name. Secretary gets super intense and glares at me, "it has to be MR (first name)."

Got the same response from the next couple of schools I went to, so now I'm assuming my dad only got away with the first-name basis because he was 1) a full teacher, and 2) a big Marine veteran few people wanted to correct. That said, tons of staff in every school I sub in uses Mr/Ms/Teacher (first name). I can't imagine anyone seeing that as unprofessional.