A third child from an alternative Perth private school has been diagnosed with measles.
The Health Department said the case was confirmed on Wednesday, more than two weeks after the diagnosis of a Year 10 student at the Perth Waldorf School in Bibra Lake.
That student became infected while travelling overseas.
At the time, the Steiner school asked Year 10 students who had not been vaccinated to stay home until August 14.
Unvaccinated students who attended a party held by the original infected student's younger brother, and students who were on a bus, were also asked not to attend school.
The department said the school had a low rate of vaccination and further cases remained possible.
But it said the broader public health risk was considered to be low, and it was continuing to monitor the situation.
A mobile vaccination clinic was held at the school on Thursday after being offered by the WA Government.
The department confirmed 15 of the school's 400 students were immunised at the clinic.
In a newsletter to parents this week, the school's acting administrator Mark Phillips said students that had been kept home from school had been welcomed back on Thursday this week.
"We wish for the best knowing that we follow the direction of the Health Department in dealing appropriately with any situation that may arise," Mr Phillips said.
He said as a result of the "unusual circumstances", the school had opted to postpone an open day that was scheduled for August 26.
