If its a guy, start with 'Mr'

Mr.Gillingham: Mr.G is fine

Viola: Can i just call you Mr.Gillingham? Because i get names mixed up easily... i thought you were Mr.Thurbur

Me: Mr.Thurbur? haha (1. there is no Mr.thurbur at our school but 2. there is a Mrs. Thurbur...)

Mr.Gillingham: :) You can't go wrong if you at least start with 'Mister'