This may look like a normal pencil, but it's definitely for left-handed people. Tell the teacher you can't do any work without one.
Then call it. It'll be impossible to answer! Unless they've got a pair of scissor handy...
Not all at once. Just you. Start with the person next to you. Then go to the back. Then to the front. Shrug when the teacher asks what you're doing.
This will take ages but will be worth it. When the teacher asks why the classroom is covered in foil, just say you want to keep the lessons fresh. They'll laugh. You'll laugh. Then you might get detention because it's such an awesome prank.
Simple, but very effective. Make sure it looks like an official sign for maximum impact.
This very subtle prank will make your teacher's mouse useless. They'll even leave the classroom to get help. That's the best time to remove the sticker and it'll be like nothing happened at all and make your teacher look silly.
If the teacher asks what you're doing, just say that your hand is full of helium. And it's free to do, so everyone wins.
Bring your pet into class and pretend it's a new student called whatever your dog's name is. Dave, call the dog Dave.
Get hundreds of cups and fill them with water and place them carefully all through the school corridors. This will cause mayhem and no-one will be able to get to their classroom without getting wet socks. Don't own up to the this prank though, as you'll be made to pick them all up!