Answer by taking a quote from the following article:

Seung-Hui Cho (in Korean, properly Cho Seung-Hui; January 18, 1984 - April 16, 2007) was a US resident of South Korean origin, a spree killer and mass murderer who killed 32 people and wounded 17 others armed with two semi-automatic pistols on April 16, 2007, at the Virginia Polytechnic Institute and State University in Blacksburg, Virginia. An additional six people were injured jumping from windows to escape. He was a senior-level undergraduate student at the university. The shooting rampage came to be known as the Virginia Tech shooting.

Cho attended the Poplar Tree Elementary School in Chantilly, an unincorporated, small community in Virginia's Fairfax County. According to Kim Gyeong-won, who met Cho in the fifth grade and took classes with him, Cho finished the three-year program at Poplar Tree Elementary School in one and a half years. Cho was noted for being good at mathematics and English, and teachers pointed to him as an example for other students. At that time, according to Kim, nobody disliked Cho and he "was recognized by friends as a boy of knowledge;... a good dresser who was popular with the girls." Kim added that "I only have good memories about him." An acquaintance noted that "Every time he came home from school he would cry and throw tantrums saying he never wanted to return to school" when Cho first came to the U.S. in about the second grade.  In 1999, during the spring of Cho's eighth grade year, the Columbine High School massacre made international news and Cho was transfixed by it. "I remember sitting in Spanish class with him, right next to him, and there being something written on his binder to the effect of, you know, ' 'F' you all, I hope you all burn in hell,' which I would assume meant us, the students," said Ben Baldwin, a classmate of Cho. Cho also wrote in a school assignment about wanting to "repeat Columbine". The school contacted Cho's sister, who reported the incident to their parents. Cho was sent to a psychiatrist.  Cho attended secondary schools in Fairfax County, including Ormond Stone Middle School in Centreville and Westfield High School in Chantilly, and by eighth grade had been diagnosed with selective mutism, a social anxiety disorder which inhibited him from speaking. Through high school, he was bullied for his shyness and unusual speech patterns. According to Chris Davids, a high school classmate in Cho's English class at Westfield High School, Cho looked down and refused to speak when called upon. Davids added that, after one teacher threatened to give Cho a failing grade for not participating in class, he began reading in a strange, deep voice that sounded "like he had something in his mouth. [...]" While several students recalled instances of Cho being bullied and mocked at Westfield, most left him alone and later said they were not aware of his anger. While in high school, the Columbine High school shooting would provide a great source of inspiration for Cho. Cho idolized the two killers. He was "compelled to replicate the Columbine boys, even out do them," which he did in terms of the number of victims  Cho graduated from Westfield High School in 2003.

How was Cho's behavior in school?
Through high school, he was bullied for his shyness and unusual speech patterns.