A van was seen to be waiting outside schools at peak times with the driver "blowing kisses" at female pupils and asking them to get in the van.
Cambridgeshire police have since launched an investigation following safeguarding concerns from parents and schools.
At least three schools in Peterborough have warned parents and pupils to be "vigilant" but parents have said they are "still worried".
Ken Stimpson Community School issued the following email to parents: "Please be aware of a white Mercedes van, which has blacked out windows.
"This vehicle has been seen hanging around a number of schools in the Peterborough area with the occupants blowing kisses and winking at young female students.
"They had also stopped and asked for the girls to get into their vehicle.
“Can we please ask that students and parents/carers are vigilant and any concerns are reported to the police via 101.”
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