I'm not a teacher, but as a native speaker, I would expect to hear a surname after "Mr". So, either "Mr. Smith", or "Mr David Smith". The use of "Mr" as an honorific with only a given, or first, name is not standard in the variants of English I'm familiar with.
It is not usual to use honorifics with the first name alone. I have met it where parents want a clear distance between the child and its tutor, and do not want the full formality of using the family name, but it is not something that will be used in normal English.
In the case of your sentence you could have argued that it was your teacher's family name
One exception being the titular honours awarded by the Head of State, in which situations, the holder is often referred to as "Sir (given name)" or "Dame (given name)". That's if I remember the protocols correctly. :-D
It's a custom in the southern United States for someone to address an elder with whom they are friendly as "Mr. Charlie" or "Miss Sally." It's less formal than using their last name ("Mr. Smith"), but it still shows respect. Elvis Presley, a good ol' Southern boy, always addressed Sam Phillips of Sun Records as "Mr. Sam."