A missing 15-year-old is believed to have run away to France with one of her teachers, police have told Sky News.
Megan Stammers is believed to be travelling with 30-year-old Jeremy Forrest - her maths teacher.
She failed to turn up at Bishop Bell Church Of England School in Eastbourne on Friday and hours later she crossed from Dover to Calais.
Sussex Police have urged either Megan or Mr Forrest to contact them.
Chief Inspector Jason Tingley said officers were at a "critical stage of the investigation".
"We are investigating several lines of enquiry and are grateful for the support and co-operation from Megan's family and friends," he added.
"Her family are incredibly worried and want her to come home."
Appeals for information about her whereabouts have been posted on Facebook and Twitter , including from her worried father, Martin.
"Have to remain strong for Meg's, can't dwell in the misery of 'what ifs' - we love you sweetheart, doing all we can," he wrote.
A website called Megan Stammers Missing has also been set up to appeal for information about her whereabouts.
Megan is described as 5ft 6in tall, with long, dark brown hair and was last seen wearing a white vest and a silver necklace.
Officers have appealed for information about the whereabouts of a black Ford Fiesta, registration GJ08 RJO, driven by Mr Forrest, from Ringmer, near Lewes.
Megan's school said in a statement on its website: "We are extremely concerned about a missing pupil and, if anyone at all has any information, we would urge them to contact the police as a matter of urgency."
Earlier, police said they did not believe she was at risk and were working with French authorities to trace the pair.
Anyone with information is asked to call officers at Sussex Police on 101.