1. Jenny's friends're not angry.
2. Your friends're generous.
(Are the contractions grammatical?)
3. Middle school is not the same as elementary school.
4. Middle school is not the same as elementary school is the same.
(Does #3 mean #4? Is #4 the full form of #3? What is the part of speech of 'as' in #3?)
Yes, they're grammatically correct, but usually used only in dialogue, not in formal writing.
3 is fine, but 4 is not. You don't need "the same" at the end. You can either keep or omit "is" before that.
In 3 and 4, "as" is being used as a conjunction.
http://www.dictionary.com/browse/as?s=t
1. Yes, the contractions in sentence 1 are grammatical. "Jenny's" is a contraction for "Jenny is" and "friends're" is a contraction for "friends are."
2. The contraction in sentence 2, "friends're," is not grammatically correct. The correct contraction would be "friends are" without the apostrophe.
3. Sentence 3 means that middle school and elementary school are not identical or the same. They have differences.
4. Sentence 4 is not the full form of sentence 3. Sentence 4 is ungrammatical as it repeats "is the same" at the end, creating redundancy. The correct form would be "Middle school is not the same as elementary school."
In sentence 3, "as" is a conjunction that is used to introduce the comparison being made. It functions as a subordinating conjunction, connecting the two noun phrases "middle school" and "elementary school" and indicating that they are being compared.