As a Canadian, you have to file taxes. Your best bet is to file for non-residency status before you leave Canada.

Americans must file as well, but your base income will likely not see you paying income tax.

In any case, you should contact your government tax agency before you go to see what they recommend.

First of all, it should have been stated in your contract specifying how much and what exactly he is holding it for.

Employers will sometimes hold a small portion of your first couple of paychecks as assurance to them that you won't leave them with any outstanding bills when you leave. It should be a reasonable amount (usually 200,000-300,000 won in total) and it is usually returned to you before you leave. These people have signed for your visa and are responsible for you while you are in Korea so it should be understandable that they want to protect themselves. Anything you leave outstanding comes back on them in the end.

You need to find out the amount owing on each bill (hydro, electric, phone, etc.) to the date you are leaving. The bill companies should be able to predict the amount in advance. Pay them all and keep a copy of the reciepts to show as proof.

Another option would be to allow for someone at the school you trust to take care of making sure you get your money back after the bills are paid. Or to have your boss pay you out completely and leave some money with that person you trust.

Start preparing yourself as your final month approaches. Ask your employer about everything you have to do to make sure you get paid everything before you leave.