The blast on Tuesday apparently targeted students boarding a bus in front of their university in the town of Maramag.
Vehicles, owned by the Rural Transit Company, have been target of terrorist attacks in recent months. The last such attack occurred last month in Mindanao, leaving four people wounded.
No group has claimed responsibility for the attack. But Philippine officials blame militants operating in the south, especially Abu Sayyaf group, for the attack.
The al-Qaeda-linked Abu Sayyaf is a small group of extremist rebels who operate in the Philippines.
Founded in the 1990s, the militant group has been blamed for the worst terrorist attacks in the Philippines’ recent history including the 2004 bombing of a ferry that left more than 100 dead, abductions of foreign missionaries and tourists, and beheadings mostly of local people.