In general, I will always expect my kids to call all teachers "Title Lastname". It's tricky this year bc ds2 is in a school where several of our friends are teachers and he has to remember not to shout out "Hey, Firstname!" I always use Title Lastname when speaking to my kids about their teachers.
I still remeber when my now 11th grader was in 2nd grade and his teacher sent home a newsletter every week signed Firstname. That was the first time EVER in my life I called a teacher by her firstname and I felt very awkward doing it, but I had ds at 17 and usually am the youngest mom in the class each year so I had major issues with feeling unequal to the teachers.
Now, though, I seem to be older than most of my younger son's teachers. He has had a few middle-aged teachers so far and I called them Title Lastname out of respect for their age, but now I am more confident and when I meet a new teacher, if she appears to be my age or younger, I say "Hi, I'm Myfirstname. You're Herfirstname, right?" and it just gets it out of the way for the year.
Last year I was on a first name basis with the principal and that was kinda cool! haha