Eight others were injured, with four in critical condition, and all the victims were students aged 14 to 17, said Albert Palencia, mayor of Banga town in South Cotabato, where the shooting took place.
The incident on Friday follows two fatal gun incidents since June at schools in the Philippines, where school shootings were previously rare.
"The suspect supposedly sat beside his friends and told them to go home 'because after lunch, I will be executing my plan'. His friends laughed because the shooter was a known joker," Palencia told Reuters.
"After lunch, he started shooting students. He hit 10 students before killing himself."
In a separate interview with DZBB radio, Palencia said the suspected gunman appeared to have fired indiscriminately at students in a classroom next to his own after leaving his grade 9 classroom.
Palencia said the suspect used a 9mm pistol belonging to his father, a Department of Education employee, after breaking into a vault where the firearm had been stored and bringing it to school.
Palencia said the teenager carried two magazines for the weapon and reloaded after emptying the first one before turning the gun on himself.
The mayor described the suspect as an intelligent student and active in taekwondo, adding there was no indication he had been bullied.
Authorities are also examining the role of online communities in the attack.
Palencia, citing information from police, said investigators found the suspect had posted in an online "true crime" community and were trying to determine the nature of the online group.
The disclosure comes after a Senate inquiry into a deadly school shooting in Tacloban in June examined the role of the so-called True Crime Community, which police cybercrime officials described as an online network of young people who share and consume violent extremist content.
At least three students were killed and about 20 others injured in that attack at a public high school in Tacloban, where two classmates opened fire on campus.
In August, a student live-streamed himself shooting and killing another student in a classroom in Zamboanga in the country's south before taking his own life, reigniting debate over children's access to social media and online content, as well as calls for tougher gun storage and safety measures.
The Philippines has relatively strict gun ownership regulations, including background checks and psychological evaluation requirements, although illegal firearms remain in circulation.
Friday's shooting also follows a high-profile incident in Thailand in August, where a student went on a shooting rampage at a school on the outskirts of the capital Bangkok.
South Cotabato governor Reynaldo Tamayo urged parents to ensure guns were kept out of reach of minors.
"We cannot expect a handful of security personnel to monitor thousands of students. We need to work together because otherwise this will be very difficult for us to address," Tamayo told Reuters by phone.
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