Louis Chadwick, from Meadowhead junior school, Blackburn, made his discovery last week on an expedition with the Dinosaur Isle Museum, Sandown, Isle of Wight.
Scientists have few examples of the small but vicious meat-eating dinosaur, called a coelurosaur, which is one of the earliest ancestors of the bird.
Steve Hutt, an expert at Dinosaur Isle, said: "This find is extremely helpful and precious because we have only five small examples of this species here.
"We are trying to find out what it looked like and what it was related to.
This is an important piece of the jigsaw."
The two-centimetre bone is half of one of the beast's vertebrae.
Coelurosaurs, whose name means "hollow reptile", lived in the early Cretaceous period. They were light, fast-moving carnivores with large heads and sharp teeth. They grew to two metres, ran on their back legs and used the sharp talons on their front legs to kill prey such as small rat-like creatures, lizards and insects.
The school has donated the find to the museum where it will be displayed in its collection of 20,000 fossils.
Mr Hutt is making a replica which the school will display in its entrance hall.
Louis said: "I thought the trip would be boring but since finding the bone I find fossil-hunting quite fascinating.
"I want to go back to the Isle of Wight with my dad on weekends."
Head Janet Knight plans to make the trip to Dinosaur Isle an annual event after this year's success.