Chalk is an item in White Day: A Labyrinth Named School (Remake).
It is found in the Faculty Office 1, 1st floor of the Main Building Sections 2 in the Mobile remake version.
In the PS4/PC version, it is found in Classroom 2-8 on 2nd floor of Main Building, Section 1 of Yeondu High School.
If you acquire both ( Chalk & Bloody Workbook ) and go to the Classroom 2-7, you can see The child who is solving a problem.
( ※ But, It is possible When you play the Hard mode or more )