Iruka was sick, because he had spent half his day drinking at the bar last night. He had alcohol poisoning and also hypothermia. So he called in two random guys with a unibrow.
"HELLO MOST YOUTHFUL CHILDREN! TODAY I WILL TEACH YOU THE HISTORY OF THE VILLAGE!" screamed Gai.
"AND IF YOU CAN'T DO THAT, I WILL DO A UNIBROW STAND!"
"AND IF YOU CAN'T DO THAT, I WILL JUGGLE FIVE MOUNTAINS WITH MY PINKY FINGER!"
"AND IF YOU CAN'T DO THAT, I WILL RUND AROUND THE WORLD ON MY EYELASHES."
This continued for an hour, until it ended with...
"GAI-SENSEI!"
"LEE!"
"GAI-SENSEI!"
"LEE!"
They were hugging with a sunset behind them and unicorns everywhere.
This continued for another hour. Finally, the lesson was ready to begin. The whole class was shivering in fear, with some children crying. Gai did not notice.
"NOW, SHODAI HOKAGE WAS KNOWN AS HASHIRAMA SENJU! HE BECAME HOKAGE AFTER HE DEFEATED THE UNYOUTHFUL UCHIHA MADARA, USING THE FLAMES OF HIS YOUTH! THEN HIS FLAMES BURNED BRIGHTER, AND THAT IS WHY THE LEADER OF THE VILLAGE IS KNOWN AS HOKAGE! ALL SHINOBI OF KONOHAGAKURE BURN WITH THE FLAMES OF YOUTH!"
Finally, the lesson was over. The next day Iruka came to an empty classroom.
"Iruka," said Kakashi. "Your students are all in the hospital after being traumatised by Gai and Lee. They are both now rated in the bingo book as SSS rank ninja." Kakashi shuddred.
"MY ETERNAL RIVAL! LET'S TRAIN!"
Kakashi and Iruka ran away screaming.