Carlos' hormones were so reinless that he can't avoid but cum on Billy, a red haired boy in the front line.
"Hey! Watch out, dumbass!" Billy complained as he cleaned his arm.
"Em, sorry Billy." Carlos replied ashamed. "I don't know why I did that!"
"Hormones." Mr Mendes interfered. "They are responsible of the changes during growing and they also increase the libido."
"What's libido?" Asked some boys.
"In a few words, it's the need to have sex." Mr Mendes said. "Like being hungry or thirsty"
"But if I'm hungry or thirsty I can wait to eat or drink something." Billy upset said. "Carlos must control his libido as well!"
"It's different." Carlos said as he saw his still hard penis. "It's not like I want you to see my penis like this."
"I could try to explain it to you but it's better that you experience it for yourself." Mr Mendes said as he used the magic book on Billy.
"Wait NO!!" Billy said but it was useless, he was already growing.
"I recommend you to take your clothes off." Mr Mendes said when he noted Billy's shirt stretching to its limit.
B-but... FIne!" Billy said in a cracking voice "But I will leave my boxers on."
"Okay, suit yourself" Mr. Mendes replied. He watched Billy standing up next to Carlos wearing just his underwear. He was an average sized boy, but stood about a foot and a half shorter than Carlos. His body showed no muscle nor hair development.
At first no one noticed anything, until someone saw that Billy was getting a little taller than before. Billy looked at his classmates and saw them slowly getting smaller. Soon, his body started puberty. He felt a tingle in his crotch. He looked into his boxers and exclaimed "HOly shIt! I'm gEtting hAir AlrEady!" Dark red hair also began to grow around his ankles. His hands and feet grew out of proportion. The remaining baby fat Billy had melted away. There was only a lean body left. Mr. Mendes glanced at how tall Billy was getting. Billy was now only half a foot shorter than Carlos, and still growing. Billy looked around the age of 13.
Then, the class watched in awe as Billy experienced his growth spurt. His boxers were now very tight on him and failed to hide his growing penis, which was experiencing its own growth spurt. His limbs continued to stretch. Hair started to sprout everywhere; his leg hair thickened while softer hairs appeared on his arms. "ThIs Is crAzy!" he shouted in his cracked voice. He put his hands on his head, timed perfectly for the class to see underarm hair grow underneath. Peach fuzz appeared on his face. Billy continued to grow, meeting and soon surpassing Carlos' height. Suddenly, a ripping noise was heard as his boxers could no longer take the growth. "Oh fuck!" Billy cried out in a deeper (but still thin reedy) voice as his privates were exposed. Everyone got to see the the pubic hair that he had developed. His dick and balls were also bigger: his dick was about 4 or 5 inches long, his balls had grown a lot in size. Billy was now about 16.
The growth in height started to slow down, but his muscles started developing. He didn't become extremely muscular, but defined. His leg muscles pushed out, his biceps enlarging. His chest heaved out, and his stomach formed some sort of four-pack. "This is so weird!" Billy exclaimed. His voice had deepened to a manly tone. Billy's height growth stopped, putting him above six feet tall, only a few inches taller than Carlos. Then, hair continued to thicken all over his body. His leg hair could now be seen from the back of the classroom. His arms took a similar appearance. His pubic hair peaked, forming a trail in the middle of his torso, meeting the dark red chest hair in the middle of his pecs. Billy felt a warm sensation on his face as its peach fuzz was replaced by stubble, his jawline squaring off. The growth in his crotch slowed down, his penis was around seven inches, his balls the size of eggs. The changes stopped.
Now, in the middle of the classroom, there stood two naked 18 year-old former boy students.