Chapter 1 hosts a good range of characters, not just the ones whose names you find on the blackboard in the room where you begin! Some of them have gotten into rooms you initially can't access. Others are somewhere else entirely. As you progress the story, a new mechanic will allow you to access the entirety of the level, so to avoid backtracking I advise not bothering to find everyone until you've unlocked the mechanic.

Time travel. Just playing the chapter through will net you this game-integral exploration tool. To get it, simply head to the Gymnasium on the 1st floor, then when your objective changes, to the 3rd floor roof access. After an interaction on the roof, the new mechanic unlocks. At this point you can access all the folks you need for achievement completion.

Despite adding her appearances to the list, I strongly recommend not speaking to Jiao until the achievement pops.

[Spoilers start here!] Names and locations will not be labeled for spoilers from hereon. Please don't ruin any surprises for yourself!
For the sake of orientation, the school is laid out in an H shape with main corridors to the West and East. There are stairwells connecting the three floors at the middle and bottom of the H shape. On the second floor, the connecting corridor is at the Southern set of stairwells. On the first floor, it's by the middle stairwells. You can also exit outdoors at the North end of the first floor to find the Yard.

??? - North - Yard - Abandoned School
Dogged Student - West - Library - High School
Librarian - West - Library - High School
Reader - West - Library - High School
Reader - West - Library - High School
Wistful Student - West - Library - High School
Smoking Student - West - Urinals - High School
??? - West - Urinals - Abandoned School
??? - West - Main Corridor - Abandoned School
Slacker Student / Lazy Student - West - Cafeteria corridor - High School
*Jiao - West - Cafeteria - High School
Business Student - West - Cafeteria - High School
Pigeon Student - West - Cafeteria - High School
Student - West - Cafeteria - High School
??? - West - Cafeteria - Abandoned School
Pigeon Prime - West - Cafeteria - Abandoned School
Hussein - East - Art Supplies - High School
VeeVee - East - Art Supplies - High School
Art Teacher - East - Art Class - High School
Law Student - East - Art Class - High School
Artist - East - Art Class - High School
Subject - East - Art Class - High School
??? - East - Art Class - Abandoned School
Science Student - East - Science Class - High School
Gross Student - East - Science Class - High School
??? - East - Science Class - Abandoned School
Keen Student - East - Main Corridor - High School
??? - East - Main Corridor - Abandoned School

Speaker - West - Student Council Room - High School
Historian - West - Student Council Room - High School
Scribe - West - Student Council Room - High School
Treasurer - West - Student Council Room - High School
Parliamentarian - West - Student Council Room - High School
President - West - Student Council Room - High School
Vice President - West - Student Council Room - High School
??? - West - Student Council Room - Abandoned School
??? - West - Main Corridor - High School
Teacher - West - Main Corridor - High School
*Jiao - West - Detention - High School
Blissful Student - West - Detention - High School
Trapped Student - West - Detention - High School
??? - Connecting Corridor - Abandoned School
Music Student - East - Main Corridor - High School
*Jiao - East - Music Room - High School
??? - East - Music Room - Abandoned School
Worried Student - East - Nurse's Station - High School
Nurse - East - Nurse's Station - High School
Nurse - East - Nurse's Station - High School
Sick Student - East - Nurse's Station - High School
??? - East - Main Corridor - Abandoned School
Actor - East - Media Class - High School
Director - East - Media Class - High School
??? - East - Media Class - Abandoned School
Older Sister - East - Toilets - High School

Johnson the 50 - Stairwell - Abandoned School
Angsty Student - Main Roof - High School
Follower - Main Roof - High School
Follower - Main Roof - High School
Follower - Main Roof - High School
Teacher - Main Roof - High School
Journalist - Main Roof - High School

I hope you enjoyed the time spent reminiscing! If this guide was helpful to you, please give it a rating. Enjoy the rest of 1000 x RESIST!