NORTHBRIDGE — A school bus had quite the scare in Massachusetts after a student's backpack burst into flames.Police say the fire happened around 7:15 a.m. Wednesday in Northbridge as the students were heading to Blackstone Valley Regional Vocational Technical High School.The bus driver was able to evacuate the students and use a fire extinguisher to put out the blaze.Authorities traced the fire to a vape pen battery that had exploded.No injuries were reported, and a second bus was able to transport the students to their destination.