I found a few versions for Boys Over Flowers - Because I'm Stupid (내 머리가 나빠서). In case you want to know how to read the Korean words in the bracket : Nae Morega Nappaseo *This is a direct translation of the pronunciation.
The 1st version, credits to Gerry Chew:
The 2nd version, credits to Ah Doe:
Because I am Stupid Piano Instrumental (Korean Drama Boys of Flower) Complete Piano Sheet
The 3rd version, credits to Ah Doe:
Because I am Stupid (因为我太傻) from Korean Drama Boys of Flower (韩版花样男子) Complete Piano Sheet (With Lyrics)
The last one, credits to Yumi:
Boys Over Flowers OST - SS501 - Because I'm Stupid.pdf
You can get the lyrics for the songs here:
BOYS OVER FLOWERS 꽃보다남자 OST ROMANIZED LYRICS
If you want to get the other music scores for BOF, please send me an e-mail at aikychien at yahoo dot com and I will send you all of them.