I've always hated the first name basis stuff. It is essentially like placing yourself at the student's level, when you are their teacher, not their peer. Conversely, adding teacher to the end just promotes Konglish over English. I'd agree with the above poster that that seems to have become the standard, although I honestly am beginning to suspect through NT choice, not the Korean system (further reading:
the Daegu email).
Every Korean co-teacher I've met bar perhaps one has disliked it for being unauthentic.
Personally, if you hate the Mr/Ms Surname basis stuff, I'd recommend going with the full Korean version: Surname - Given Name -
sonsengnim or plain "teacher". Recall though that Mister doesn't carry any age connotations except that you are aged 18/21 or above, depending on nationality (prior to that you are technically a Master, still Mr). I strictly use Ms. to refer to female Korean staff in front of the students as I feel it is the least likely to cause offence.
Other alternatives exist too. I've heard "Coach" used before in non-PE settings. I've also heard abbreviations such as "Mr. K" and "Ms. P".
I'd avoid "sir" as in the Commonwealth, it can be seen as patronising to the point of being blatantly offensive and you don't wish to risk placing a student travelling abroad in an awkward situation.