You were watching two teachers duel to show the class how to defend themselves. They then picked two students to duel against each other, so they picked you and James Potter, this was great since James was your enemy. Severus Snape was in the crowd as he kept his eyes on you, you and James both prepared and when you both heard

"Go!"

You instantly shot a spell at James, causing him to fly back as everyone gasped in awe, especially Snape

"Damn.."

Snape muttered under his breath