Year 10 student Raymond Hodgson and groundsman Ben Smith unearthed six vertebrae from the ichthyosaur on May 20, while laying out a vegetable garden at the Richmond State School, in the state's northwest.
Mr Smith said Raymond was complaining about the difficult digging after he had struck something hard.
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Student 'not taking any notice'
"He hit a rock, and once he got it out, he threw it to the side because he wasn't taking any notice," he told AAP.
"I picked it up and had a look at it, and thought 'That looks more like a fossil than a rock'."
Students and the principal descended on the patch and watched as five more vertebrae were unearthed.
"Of all things in the school ground, you don't expect to see that," Mr Smith said.
Cross between dolphin and shark
The curator of local fossil museum Kronosaurus Korner, Paul Stumkat, said ichthyosaurs were prolific in the area from the end of the Jurassic period through to the early Cretaceous period.
"They were the wilderbeasts of the Cretaceous inland sea," Mr Stumkat told AAP.
The creature looked like a cross between a dolphin and a shark and could grow to about eight metres.
They had some of the largest eyes in the animal kingdom, allowing them to dive to the dark depths of the inland sea to hunt squid.
The ichthyosaur found at Richmond State School came from a mature adult and Mr Stumkat is hoping its skull will be found.
Area renowned for fossils
"To find fossils is great, but to find a complete animal is better," Mr Stumkat said.
The Richmond area is famous for fossils.
Mr Stumkat said a "grey nomad" was travelling just outside of the town in May last year, when he came across a fossil by accident.
"He went to relieve himself, and he came across an ichthyosaur," Mr Stumkat said.
"As we were excavating, it disappeared halfway along so we were suspicious that something bit the whole thing in half."
He said many of the museum's 500 specimens had bite marks, mainly from ichthyosaurs and kronosaurus - a marine reptile that was, arguably, the largest in the world.
The ichthyosaur fossils found by Raymond and Mr Smith are on display at the school and the veggie patch has been relocated for now.
