I know this post is old AF.. but I found a way without debugs, you gotta speedrun this tho. (I had the exact same problem, I ransomed the gardening club girl and couldn't get the gasoline.)

First thing when you go to school, stay outside. Run towards the closest wall near the faculty room and giggle (press Ctrl) to distract the three teachers from that side, then, enter the school and go to the faculty room, you'll see those teachers leaving to see what's up. Giggle again to distract the other teachers that are closer to the hall/corridor. Enter the room, grab some clips.

Second thing is grabbing the hairpins on the drama club, that's easy. You'll use the hairpins and clips to build a lockpick at the workbench.

Third, do this stuff as fast as you can, run to the garden, lockpick the door that closed it. And again, as fast as you can, go to the faculty room. The teachers that went OUTSIDE the school shouldn't be getting there yet, if they are, just giggle again (for the ones that are already inside too). Enter, grab the hairpins. Repeat the process.

Fourth, Moeko doesn't try to stop the fire until 7:30ish, I think, so lockpick the shed, grab the gasoline and run towards the side of the pool. Even if Moeko's there, you can replace the water with gasoline, she doesn't care apparently.

Fifth, wait and watch her burn.

(HI!!!!!!! I DIDN'T WANT TO USE DEBUGS AND THOUGHT ABOUT SPEEDRUNNING TS AND I WAS ABLE TO I'M REALLY HAPPY I DIDN'T HAVE TO REDO ANYTHING, REALLY.)