Megan Stammers, aged 15, is thought to have gone missing with male teacher Jeremy Forrest, 30, from her school Bishop Bell Church of England.
News of her disappearance came on Friday as the pupil failed to turn up to school.
Police are believed to be checking CCTV and it is understood Megan boarded a ferry on Friday night at Dover set for Calais in France.
The teenager, who is from Eastbourne, is suspected of travelling with Mr. Forrest who is reportedly married.
There have been appeals for the girl to contact her friends and family who are worried about her, with several comments being put on social media sites.
Megan’s sister Brooke tweeted: “Keeping hope that we'll have good news soon. Need our meg home safe and sound #keepfaith #findmeganstammers."
Authorities in France are assisting with the case and according to the BBC Chief Inspector Jason Tingley, from Sussex Police, said: “We are investigating several lines of inquiry and are grateful for the support and co-operation from Megan's family and friends.
"We are at a critical stage of the investigation."
Some reports confirm that police are urging the student to contact her loved ones.
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