"School Days" is the very first song of the Season One episode, "Cartman Gets an Anal Probe". School Days was written in 1907 as sheet music and recorded by many artists over the years.
Background
The boys Eric Cartman, Kyle Broflovski, Stan Marsh, and Kenny McCormick are introduced at the Bus Stop singing this song when Ike Broflovski runs up interrupting them.
The scene was recreated in the Season Seven episode, "Cancelled". This version had the boys saying and acting out everything from the first episode except in a more profane manner.
Lyrics
Portion of the song heard:
School days, school days
Teachers Golden Rule days
Full version of the chorus:
School days, school days
Dear old Golden Rule days
'Reading and 'riting and 'rithmetic
Taught to the tune of the hick'ry stick
You were my queen in calico
I was your bashful, barefoot beau
And you wrote on my slate, "I Love You So"
When we were a couple o' kids