Should I (assistant professor) call undergraduates by first name or Mr/Ms
It is disrespectful to demand more respect from your (adult!) students than you give them. So you should either both go my first name or both by last name. At least this is true in by culture (Central Europe) - I would think it is the same in Northern America.
So, unless it is a cultural thing everyone does in your area to call younger people by their first name, by respectful and call them as you want to be called.