"I am in the SAME class AS Jeter." Some books explain that after the word "same," the word "as" is a relative pronoun that means something like "which." (It refers to "class.") So your sentence means something like: I AM in the SAME class which Jeter IS in. I am in this class. Jeter is in this class. We are in the same class.

Same as.I don't know the reason.it is idiomatic.I think