In 1922, Kahlo was accepted to the elite National Preparatory School, where she focused on natural sciences with the aim of becoming a physician. The institution had only recently begun admitting women, with only 35 girls out of 2,000 students. She performed well academically, was a voracious reader, and became "deeply immersed and seriously committed to Mexican culture, political activism and issues of social justice". The school promoted indigenismo, a new sense of Mexican identity that took pride in the country's indigenous heritage and sought to rid itself of the colonial mindset of Europe as superior to Mexico. Particularly influential to Kahlo at this time were nine of her schoolmates, with whom she formed an informal group called the "Cachuchas" – many of them would become leading figures of the Mexican intellectual elite. They were rebellious and against everything conservative and pulled pranks, staged plays, and debated philosophy and Russian classics.
She fell in love with Alejandro Gomez Arias, the leader of the group and her first love. Her parents did not approve of the relationship. Arias and Kahlo were often separated from each other, due to the political instability and violence of the period, so they exchanged passionate love letters.
On 17 September 1925, Kahlo and her boyfriend, Arias, were on their way home from school. They boarded one bus, but they got off the bus to look for an umbrella that Kahlo had left behind. They then boarded a second bus, which was crowded, and they sat in the back. The driver attempted to pass an oncoming electric streetcar. The streetcar crashed into the side of the wooden bus, dragging it a few feet. Several passengers were killed in the accident. While Arias suffered minor damages, Kahlo had been impaled with an iron handrail that went through her pelvis. She later described the injury as "the way a sword pierces a bull." The handrail was removed by Arias and others, which was incredibly painful for Kahlo.
Kahlo suffered many injuries. She spent a month in hospital and two months recovering at home before being able to return to work. As she continued to experience fatigue and back pain, her doctors ordered X-rays, which revealed that the accident had also displaced three vertebrae. As treatment she had to wear a plaster corset which confined her to bed rest for the better part of three months.The accident ended Kahlo's dreams of becoming a physician and caused her pain and illness for the rest of her life.
Kahlo's bed rest was over by late 1927, and she began socializing with her old schoolfriends, who were now at university and involved in student politics.