These are all the possible shield locations that I am aware of...

Glass Screen:
-Cafeteria, leaning against a pillar
-Speed Cola Basement, to the left of the Ray Gun MK2 crate
-First room on the left when you enter the school, to the right of the green register screen
Eagle Head:
-Main Courtyard, against a cthulu stachu
-Leaning against some barrels in the forest area, next to the AK47 wallbuy
-To the right of the Headmaster's Office, leaning against a radiator
Engine:
-Leaning against a crate, go left immediately once you open the door leading to the cabins
-In the room with Madgaz Moonshine, next to a sideways locker
-In the labs, leaning against a wall near Double Tap 2.0

Workbench
-In the cabin closest to Banana Colada, in the forest area