Police said the attack happened in front of the entrance to the Darul Himah School in Pu-gnoh village in tambon Kaliza at 11:20 am.
Seven troops were travelling in two pick-up trucks to the school to meet local residents as part of their psychological operation when insurgents detonated the bomb with a walkie-talkie.
Sgt Maj 1st Class Ekkapop Padphu, 33, the driver of a pick-up, died of multiple wounds caused by bomb shrapnel.
The three others were severely injured and rushed to the Ra-ngae Hospital.