As I know, not quite often. But when Brian was in K2 I noticed that his native teacher always not around cos she was pregnant & always have leave. It's quite ok for his K3 native teacher. Their local teachers are stable. Anyway, I think it's ok to change the teachers for every new level but not half way during the school term.
I will be out of town on 11th Feb to 15th Feb. So, when will you wish to have the gathering?