Beijing, China (CNN) -- China's kindergartens and preschools increased security after another violent attack on children and teachers Friday.

The execution of a former doctor who stabbed eight children to death and wounded five others at an elementary school in eastern China last month failed to deter what police say are copycat attacks, state media reported.

"Chinese society has generated enormous pressure on individuals and some of those individuals have perhaps had emotional and psychological problems," sociologist Ding Xueliang said.

"They want to cause general attention from the population and attacking kids perhaps is the best way from their perspective of achieving this objective."

The latest attack came Friday, when a man armed with a hammer injured five preschool children in east China before setting himself on fire in a classroom suicide, a government spokesman told Xinhua news agency.

The attacker held two children in his arms as he poured gasoline over himself, the spokesman said. Teachers in Weifang City, Shandong province, pulled the children away as the man died, the spokesman said.

All five victims from the attack were hospitalized with injuries that were not life-threatening, he said.

The incident followed at least three other attacks in China in recent weeks in which assailants have killed or wounded students.

On Thursday, at least 28 children were injured when a man with a knife attacked a kindergarten in east China, state media said. Most of the victims were 4-year-olds and three of the children were in critical condition.

Police said they have arrested a 47-year-old suspect. The incident happened in Taixing city in Jiangsu province.

A day earlier, a man attacked 18 students and a teacher with a knife at a primary school in southern China's Guangdong province, Xinhua said.

It was not immediately known how many people were injured or whether there were any fatalities in the attack Wednesday. The man, described as being in his 40s, entered the school in the afternoon and attacked before he was seized by police, Xinhua reported.

This week's attacks came despite the execution of Zheng Minsheng, 42, a former community doctor convicted for the March 23 attack.

Zheng, executed by a firing squad in Nanping City on Wednesday, told investigators he carried out the attack because he was frustrated by "failures in his romantic life and in society," according to Xinhua.

China Daily newspaper quoted Nanjing University sociology professor Zhu Li saying Zheng's attack inspired copycats.

"Some people may not have thought about stabbing school children, but due to the media's coverage of such a case, they got an idea," Zhu said.

Chinese authorities have begun teaching safety awareness in school curriculums, China Daily reported.

Officials also have tightened security in schools by hiring extra guards to escort students to and from class.

Students, teachers and parents are receiving counseling to help deal with the trauma, according to authorities.

CNN's Jaime FlorCruz contributed to this report.