The remote-controlled bomb was triggered by a mobile phone signal at a bridge in Yi-ngo district when a six-wheel truck carrying students was passing.
Two young women - 14-year-old Seeteenuriya Baga and 17-year-old Nadia Jehama -- were wounded and were rushed to hospital.
Driver Aekaphong Samu, 58, said he was picking up 70 students to send them to school as usual. His truck was escorted by a Thai army school security escort team in a pickup truck when the incident occurred.
Following the incident, Narathiwat's senior education director Terdsak Tavornsuth visited the school and met with the Romklao school principal. They later decided to close the school for two days. Parents came to collect their children, and take them home. The school will reopen next Monday.