To unlock the School Kids Achievement you'll need to collect all of the Glitched Remains that you can find during Chapter 2 (The School chapter). Here is my video guide that shows how to get all of the Glitched Remains that are in Little Nightmares 2. The timestamps are just below the video. If you're just looking for the ones in Chapter 2 then just follow the timestamps.
Chapter 1Glitched Remains #1 - 0:23
Glitched Remains #2 - 0:47
Glitched Remains #3 - 4:51
Glitched Remains #4 - 6:35
Chapter 2Glitched Remains #1 - 6:56
Glitched Remains #2 - 7:18
Glitched Remains #3 - 8:34
Glitched Remains #4 - 10:24
Glitched Remains #5 - 10:41
Glitched Remains #6 - 11:26
Chapter 3Glitched Remains #1 - 11:45
Glitched Remains #2 - 14:02
Glitched Remains #3 - 16:07
Glitched Remains #4 - 16:15
Chapter 4Glitched Remains #1 - 18:42
Glitched Remains #2 - 19:22
Glitched Remains #3 - 19:44
Glitched Remains #4 - 20:25