Here’s how to finish the Riddle School 2 flash game.
1. In the band room, click on all the coins. There’s one on the drum set, two on the floor, and one on the music stand closest to you.
2. Talk to the guy leaning in the left corner of the room, and you’ll buy a whistle from him for 75 cents.
3. Click on the whistle to blow it, and the teacher will faint. Exit the room.
4. In the hallway, enter Mrs. Sleep’s room on the left.
5. Click on the open student desk to get SLIDY trombone lube and a few coins. Leave.
6. In the hallway, go 2 rooms to the right. Click on SLIDY and the vent will open. Pick up the toilet paper.
7. In the hallway, go into the teacher’s lounge. Click on the first conversation option and get ejected.
8. In the hallway, go one room to the left and click on the first locker in the set on the right. Get the coin at the bottom when it opens.
8. Go into the Men’s Room. Click on the toilet paper. After the animation, get the mop. Leave.
9. In the hallway, go one screen to the left, and into the Janitor’s Closet. Before you click his speech bubble, click on the mop in your inventory first. He gives you money. Leave.
10. In the hallway, go one screen to the right. Go into Mr. Sum’s class which is now open. Click on the papers on his desk, and then get the coin that appears. Leave.
11. In the hallway, go all the way to the right, and into the teacher’s lounge again to find it empty.* Click on the wall behind the bottom of the cookie machine, and click on the cord head to plug it in.
12. Click on the cookie machine to buy a cookie. Leave.
13. Click on the cookie in your inventory. After the animation, click on the Office door. When inside, go thru the door on the right. After you’ve got the key, leave the office.
14. Go all the way to the door on the far right of the hall, and you’re done.
* The action of getting ejected the first time from the teachers’ lounge, and travelling all the way to the left end of the hallway is what clears the teacher’s room (which I suspect is to negate an animation otherwise necessary to show them walking past you).
I also made a short video of the puzzle being solved visually..