Tomorrow is my daughter's last day as a shift manager at our local coffee shop. There was no resignation letter. Technically, she is not resigning --she's just not going to be on the schedule since she'll be in Vancouver for her second year of college.
The rule is as long as she works once every six weeks, she maintains her employee status. Last year, her manager called her every six weeks and paid her a couple of bucks for the call to maintain her active status. That way, she was able to slide right back into the job without a lot of fuss.