The incident took place near Mohanpur village when the bus with 40 children in it was on its way to school, a police spokesperson said.
"The 18 injured students have been admitted in a private hospital here, while condition of the bus driver Sunil and his assistant Nikku is stated to be critical and therefore, they have been referred to PGI, Rohtak," police said.
There was heavy fog and the truck tried to overtake the bus, school principal V K Mishra said.