Several western Sydney high school students have been injured with a hypodermic needle that NSW Ambulance says a fellow student found at a bus stop.

Two Plumpton High School students were taken to Mt Druitt Hospital, while several others have been sent to medical centres with their parents to undergo urgent blood tests.

The students could now face a wait of several months to complete a full round of blood tests.

Hypodermic needles are typically used on the end of syringes.

Paramedics and police were called to the school just before midday, to find several students had sustained injuries to their hands and legs.

Acting Inspector Shane Rolls described the injuries as "minor".

"While this is very concerning behaviour, we want to reiterate that no students were stabbed as initial inquiries first may have led people to believe," he said.

"At this stage we would believe it was a prank gone wrong.

"We will certainly be looking into if there was any malicious intent in this incident."

NSW Ambulance claimed the alleged offender picked up the syringe from a bus stop, however Acting Inspector Rolls said that was still being investigated.

"If that is the case, it is concerning and it's concerning for everybody in our community," he said.

Police warned they would pursue the matter further if any ill intent was established.

Wait for results 'quite concerning'

Joe Ibrahim from NSW Ambulance said the risk was "quite low".

"The puncture sites … were sort of quite minimal," he said.

Mr Ibrahim compared the tests to when ambulance officers are potentially exposed to disease risk on the job.

"The first blood test is a baseline test and you do worry about that for up to three months, it's quite concerning," he said.

The school held an assembly to do a headcount of students, according to NSW Ambulance.

The school is on the corner of Hyatts and Bottles roads, about 50 kilometres from the CBD.

Parents began to pick up their children early after reading about the incident on the internet.

"I'm concerned, that's why I want to ask is my son OK or not," Nadia Baig said as she arrived.

"I just read from the internet the news and my friends called me."