You need a little more weight in the nose. Try a paper clip or some clay. Get your plane gliding well straight ahead. The airplane model in a wind tunnel is held still and we blow air over it. Flying has the airplane moving through the air.
Gliders are light.
Sometimes you need to add a little weight to make them fly level though. Make sure they glide nice and level first. Our first redesign has too much wieght in front. If you can't remove some of the weight, try moving the wings toward the nose a bit. Your teacher can help with that.