Here are the correct answers and their explanations.
1. The correct answer is "since." "Since is correct based upon the fact that the speaker is referring to a past relationship. The word "since" refers to a passage of time.
2. The correct answer is "never." If the speaker had been to the school, he or she would know where is was located. Given the speaker has not been to the school, he or she is asking where it is located.
3. The answer is "for." Here, the answer relies upon time again, but it does not refer to time being passed already. It speaks more to the past and the present. A specific time has been given.
4. The answer is "already." Given that the "he" is smart, one can infer that he has already passed the tests given. A person who is not as intelligent would never pass the exams.
5. The answer is "just." Similar to the explanation provided for answer #2, the speaker would not have asked if the teacher was nice if he or she had just met him. Instead, based upon the meeting, the speaker is stating that he/she has already made a judgement on the teacher's niceness.
6. The answer is "already." Given that the word "gone" (past tense of go) is used, one must keep the tense of the sentence. On the other hand, some people may state that "since" is correct. This would be appropriate if the a statement had been previously made regarding another action. (For example: Although I had decided to take off a year of school, I have since gone to school.)