When Justin signs the blank piece of paper, he signs it "Justin E". Later on, he tells Michael that his last name is Polznik.
Dwight and Angela insist on staying until 5 o'clock, but when the job fair group comes back to the office, they're gone. The job fair group left the high school right before 8th period (as noted by the announcement), so it couldn't have been later than 4 o'clock and, as Pam noted when she called Jim, the high school was only a 20-minute drive from the office. Response: There is nothing to indicate the group went back to the office immediately from the end of the job fair. There could have been other stops, such as getting something to eat.