Over a dozen schoolchildren were injured, three of them critically, in Uttar Pradesh's Mainpuri district on Monday when a speeding truck hit their school bus from behind, police said.The three seriously injured children were rushed to the Saifai Medical College.The bus was ferrying 40 children who were returning from a Christmas function at Baba International School in Ghiror when the accident happened.Police say the truck driver was driving at a high speed and the fog also played a role in the accident. The truck driver fled the spot.After first aid, some children were discharged from medical facilities and handed over to their parents, an official said.