Click the pencil sharpener on the teacher's desk to slip out of the classroom.

In the hall, go east.

To the right of the drinking fountain is a locker with 2 slits; click it to open it. Inside is a hall pass. Take it, then go north into the classroom.

Click the trashcan to find a feather duster, then leave by going south.

Go east.

Go north, into the janitors closet.

Click the feather duster in your inventory to return it; the janitor will give you a dollar.

Go south, then east twice.

Click the hall pass to get past Richy, then continue east twice.

Click the dollar to give it to the kid blocking the teacher's lounge, then enter by going north.

In the teacher's lounge, click the speech bubble and choose "I need to talk to the principal", then "I want to tell him a joke"; any other option will lead you to being kicked out of the office.

After the cutscene where you steal the school key from the principal, click the teacher's speech bubbles to leave.

Out in the hall, go west 7 times to unlock the school doors and end the game.