IT CAN BE AVOIDED.
If you use your debit card or credit card to pay for a lot of stuff, then you have probably worked up a large deductible amount. I sat down with my co-teacher and she did it all for me on the 'calculate your tax deduction' website, sent the results of that to the school administrator, and it's cut my tax payment from 300 000 to 30 000. And that is freaking amazing.
Be sure to click each category (medical, credit card, debit card) so that the amount you spent appears, before downloading the pdf with all your info to send to the school admin. If you've had a lot of hospital care because you're a clutz, well, this is where karma is coming back to give you a hug.
I would post a tutorial here, but my co-teacher did it so fast I didn't really see what she was doing. Ask your co-teacher or a friendly co-worker to help you. All you need is your ARC number and the NEIS ID certificate or your ID certificate from your bank.
The website is here (best accessed on the dinosaur that is Internet Explorer): http://www.yesone.go.kr
So, that's how you can cut your taxes down right now. But for future planning, there's a little more you can do. There's a tax save card that most Koreans have, which you can register for. Then, every time you pay for something with cash, you swipe the tax card or provide the number (it might be tied to your phone number...?) and logs it with the tax authorities. You can then claim on that money too. That includes paying your bills and paying your rent. The (Korean only) website is here: http://www.taxsave.go.kr
Of course, if you're a smart Saffer who got a certificate of residence in SA and gave it to your school when you were hired in the first place you won't be in this situation but this might help people who are not from tax-exempt countries to at least make this a little less painful.