A woman wanted by police for trying to snatch schoolchildren on Brisbane's southside has posed as a grandparent in a classroom and forced a second school into lockdown.

Investigators don't know what her motive is, but they are desperate to find her before she strikes again.

The imposter was among 500 grandparents who were invited guests at Camp Hill State School on Friday.

A woman wanted by police for trying to snatch schoolchildren on Brisbane's southside posed as a grandparent in a classroom and forced a second school into lockdown. Source: 7 News

Gen Phillips was in her granddaughter's grade two classroom when she spotted the stranger with the six-year-old.

The grandmother told 7 News she heard the student's name called out, and turned around to see the little girl posing for a photo with the unknown woman.

"I never seen this woman before. That's why I alerted [the school] because I don't want anyone to have their lives taken," Ms Phillips said.

"She went and put her arms around my granddaughter! And I felt so bad I thought 'Oh my God, what's going on here?'"

The woman (pictured) was an imposter among 500 grandparents who were invited guests at Brisbane's Camp Hill State School. Source: 7 News

Ms Phillips glared at the well-dressed woman until she left, but didn't want to cause a confrontation in the classroom.

The grandmother searched the school for the woman, but when she couldn't find her, she raised the alarm.

Police were called and started patrols.

Gen Phillips (pictured) was in her granddaughter's grade two classroom when she spotted the stranger with the six-year-old. Source: 7 News

An hour and a half later, the same woman was spotted on the grounds of Whites Hill College. A staff member chased her away and the school was placed into lockdown until all students could be accounted for.

Investigators believe it's the same woman they released a picture of two days ago, wanted over two February incidents in Runcorn and MacGregor, when she tried to lure three students into her Silver SUV by telling them she was a teacher.

The woman is described as Asian in appearance, proportionate build with black hair and brown eyes.

The vehicle has been described as a silver SUV similar in size to a Toyota Rav 4.

Police are advising children to be careful when walking to and from school and to be aware of their surroundings and not accept lifts from anyone that is not known to them.

The Department of Education told 7 News it would work closely with police, but did not say whether it would boost security.

Anyone with information is encouraged to call Crime Stoppers 1800 333 000 or use the online reporting page.