The accident took place in Haldi town when the truck ploughed into the bus of Sacred Heart Public School.
"The collision was an intense one after which the school bus overturned. With the help of the locals, the students trapped inside the bus were rescued," police inspector Manoj Kumar told reporters in Ballia, some 300 km from Lucknow.
"The injured students are being treated at a hospital, where the condition of four is stated to be serious," he added.
"The injured students are in the age group of 8-12 years," he added.
Police are yet to nab the driver of the truck who escaped after the accident.
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