The original IQ tests were used to gauge what was termed "mental age", what age you were functionally acting at, and was a ratio of your mental age over your chronological age times 100--a ten year old working at an eleven-year old's level had an IQ of 110.

That said, modern IQ doesn't use that definition, and all of the material you can easily find on the matter pretends that it does. Instead, IQ is a normalized bell curve, every 15 points is another standard deviation on the bell curve. Given that definition, a year's worth of education is probably guesstimated at roughly one standard deviation of IQ. This comes with the massive caveat that IQ and educational achievement can't be compared in that manner (since one is an absolute scale and the other is definitionally a statistical artifact).