Hey, very sorry for the late response I was inactive for a long moment.

There are only 3 glitches that you can't use in a Glitchess run. The locker skip, the crouch stair glitch, and the cutscene crouch skip (with the phone), of course, if new glitches are discovered, you still can't use it. It's glitchess.

For the Locker skip, you need to enter the locker, advance to trigger the event, and when you're near the bottom of the locker (you hear a little noise when you trigger the event), then you go back and it will make you clip through, you may need some attempts to do it.

Enjoy.

Je vais le prévenir sur Discord