"I saw Aitzaz trying to get hold of a guy and then there was a big explosion," said Habib Ali, who is a senior teacher at the school.
The target of the bomber was the morning assembly of approximately 450 students, Ali said.
"He was an average student, but was a bold child," the teacher said.
Today, in a statement Pakstan's Prime Minister Nawaz Sharif recommended him for the highest civil award for bravery.
"Shaheed Aitizaz's brave act saved the lives of hundreds of students," the prime minister said in a statement.
Thousands of people thronged his home since the incident to praise his valor and to offer condolences to his family.
"He has left his family grieving, but has prevented many others from that fate," said his teacher, a sentiment echoed by his father earlier.
If it wasn't for Hasan's act of valor and quick thinking, the results would have been catastrophic.
Hasan is Pakistan's latest student to become a hero for standing up to terrorists.
Malala Yousafzai was 14 and an outspoken advocate of educating girls when a Taliban gunman boarded her school bus and shot her in the head and neck.
She survived and wrote a book "I Am Malala: The Girl Who Stood Up for Education and Was Shot by the Taliban" and has become a worldwide icon for her defiance.
Pakistan's schools are regularly targeted by terrorists and school enrolment figures have been dwindling.