There are three things that will be deducted from salary.

The tax rate for teachers in Korea (foreign and Korean) ranges from 3% to 7% and the rate depends on how much you make money monthly.

If you make 2.0 million won per month : tax rate is around 3%~5%
If you make 2.2 million won per month : tax rate is around 4%~7%

Employers are required by law to deduct from your salary.
You can not request exemption from the pension scheme.
There is each reciprocal arrangement with each country (USA. Canada. etc)
After the employee goes back to home country, the employee can get refund of their own and school's money as well.

A. National Pension Corporation (NPC)
: http://www.npc.or.kr/eng/g-index.htm

It's not obligatory one.
You can negotiate with your school to be exempt from the medical insurance.

If you have your own comprehensive travel insurance,
you must make sure it  covers you in a working situation.

The institute will enroll you in the Korean National Medical Insurance Union. The cost is split 50/50 between you and your employer, but it is voluntary and you don't have to do it. The rate is currently total 4.31% (2005 year) - 2.15% by teacher and 2.15% by school.

A. National Health Insurance Corporation (NHIC)
: http://www.nhic.or.kr/wbe/wbea/1173011_721.html

B. AIG Insurance Company
: http://www.aiglife.co.kr

This deposit is to cover any broken part of housing, unpaid monthly services, utilities, telephone charges, etc when a employee leaves the housing. The employer'll refund it if there's no harm in the housing or no bills the employee has to pay back.