Teach students how to make a basic paper airplane. If they know already, let them fold their way and start testing immediately. Make sure students make the creases tight and neat as they can.
(Plane design ideas with step by step instructions at many websites, including http://www.funpaperairplanes.com, https://www.exploratorium.edu/exploring/paper/airplanes.html, www.origami-kids.com)
Ask students to think about the forces on their planes as they fly them. Students can alter their models now if they want, but for now, the emphasis should be on watching them fly and thinking about the forces involved.
After a while, stop the flying action, gather as a group and discuss what forces they think keep the plane in the air. Introduce terms and round out the concepts as appropriate for the grade level:
1. Thrust is the forward force on an airplane. Your arm muscles throwing the paper airplane generate initial thrust which send the paper airplane forward. (In a real airplane, thrust is from a propellor or jet engine and is continually acting as the plane flies.)
2. Drag is the force that slows the plane down as it pushes against the air it is moving through. It is a kind of friction and is also called called air resistance. It acts in the opposite direction from thrust. Thrust must be equal to or greater than drag for a plane to move forward, hence your paper airplane slows down as the energy from the initial thrust is used up. The more streamlined an object is, the less drag it has. (Airliners retract their landing gear between take off and landing to reduce drag on the landing gear which would otherwise rip it off.)
3. Gravity is the force pulling the plane downwards. It acts on the mass of the plane, giving it weight.
4. Lift is the force that pushes the airplane up, acting in the opposite direction of gravity. Lift is produced by the wings and the air flowing around them (either the wing or the air must be moving - they just need to move relative to each other).
The tilt and the curve of the wing means that air flowing off the wing flows downwards. This downwards force pushes back up against the wing, and lifts the wing. This is also known as Newton's 3rd Law of action and reaction. (The Bernoulli effect has previously been used to explain lift, but is now known to be insignificant, or even incorrect - it doesn't explain how a plane can fly upside down.)
As an aside, drag and lift can be felt with a hand out of a car window:
Holding your palm flat against the wind you can feel the air pushing against it: drag, or air resistance. If you make a fist, your hand is smaller and there should be less drag on it.
If you hold your flat hand straight out of the window, then slowly tip it so the front edge is tipped up a bit, the air is directed downwards and pushes your hand up. The effect is quite dramatic and nicely demonstrates action and reaction.
Continuing the lesson, ask students how they might make their plane go further.
e.g. more thrust by throwing more strongly (they are probably doing this anyway); more streamlined to reduce drag (change how it is folded so it has a narrower front), more lift by changing the angle of attack (point it upwards to start). Tips from www.exploratorium.edu/exploring/paper/airplanes.html: If the nose drops and the plane dives into the ground, bend up the back of the wings. A little bend goes a long way. If the nose rises first and then drops, the plane is stalling. Bend down the back of the wing. Keep your adjustments small.
You may also want to discuss adding other folds, such as winglets, to their plane. Winglets are additional upwards folds on the end of the wing, and reduce a vortex of air that pushes the wing down: https://www.youtube.com/watch?v=CnvIf3vFEYA.
Flaps on the back edge of the wing can also increase lift.
Encourage students to share designs and tips on how to throw their plane, and allow more time for testing.
They can measure and record how far their plane goes. Generate a class graph of the distances achieved, and discuss factors that might increase flying distance.