As was seen at the start of the episode, Torchwood knows about the school and has closed down all easy access to the files, even with Mickey's Doctor-based advantages. So, their next logical step would be to send in an investigator.
Is Sarah Jane Smith working for Torchwood?
She has the investigative background, the knowledge of alien tech, and the drive to find out. And what sort of person brings the robot dog along just in case?
Pats companions that actually stayed on Earth in this time would be Tegan, Liz Shaw and Jo Grant. Ex-UNIT members too, possibly? Hmmmm...