Typically, the main egress door.
Not in the school district I am presently working in.
All of the power off buttons are a flush mounted style button (not a mushroom or extended head) none are located near any egress door.
Actually the welding and auto shop both share 1 button for the two separate shops, it is inside of the locked instructor's office.
The wood working shop has the button close nippled to the power panel, located in the front of the room near the instructor's station.
Some day I will ask someone where the 'Off' button is in the chemistry labs.
Our industry casually uses the term 'emergency', to the point where it is all but meaningless as an adjective.
NFPA79 has a pretty good description of Stopping and E-stopping functions.
These 'room power off' buttons do nothing to actually E-stop equipment. The individual pieces of equipment need to be protected just like on any jobsite or factory, typically to OSHA standards.
Regardless, the wiring diagram supplied by the OP is unlikely to provide the type of control we all concur is likely desired.