Bob Kane probably stole other kids' homework and put his name on it.
They probably all were zoned for the same school because they lived in the same tight-knit Jewish community in New York. "Stan Lee" is, of course, really Stanley Levy. "Bob Kane" is Bob Kahn. And Bill Finger was born Milton Finger.
So, since they were all the same ethnicity (which, in New York, closely coincided with living in the same homogeneous ethnic neighborhoods), it would be like having three Italian-Americans and marveling at the fact that they all went to the same Catholic school.
Now what would really have been amazing is if a famous Italian, Jew and African-American all graduated from the same school (because, of course, in the New York of that time-period they would have all been in radically different areas, and zoned for totally different schools).
Paul McCartney and George Harrison went to the same high school too.
Also - Will Eisner. Eisner became successful and hired many artists to draw comics. Bob Kane and Will Eisner worked for Iger. Then, Finger joined Bob Kane's nascent studio in 1938 after having met Kane, a fellow DeWitt Clinton alumnus, at a party. I'd like to know who taught art and English there in the 1930's, because they are responsible for creating the comic creators.