Three teenagers, Shamima Begum, 15, Kadiza Sultana, 16, and Amira Abase, 15, left on 17 February while they were on a half-term break, taking a flight from Gatwick to Turkey, which borders Syria.
The girls were described as "straight-A students" and had been studying for their GCSEs at the Bethnal Green academy in east London.
The families' laywer, Tasnime Akunjee, told BBC News the girls had contacted their families weeks to say they had been married to militants some weeks ago.
"The families learned that the girls were not together in Raqqa three months ago," said Akunjee, "and it was because two of them were married.
"[The families] are distraught, because it doesn't bode well for their return. The girls are starting to grow roots socially, and deep roots."
In March CCTV footage emerged which appeared to show the three teenagers at a bus station in Istanbul.
They then boarded a bus to Gaziantep, a Turkish border town which has been used by numerous foreign fighters traveling to Syria.
The pair has had husbands "approved" for them by jihadists, with one of them phoning and another used a social media to tell their families, according to The Guardian.
A video released yesterday by The Telegraph is said to show the schoolgirls shopping for groceries and dressed niqabs in the Islamic State stronghold of Raqqa.
The footage, said to be filmed by activists, purportedly shows two of the teenagers with a third women "handler".
But Akunjee said the video is unlikely to be them. He said: "The girls were not together at the time of the video," he said. "It's unlikely that they would have been under the aegis of the leader of a woman's centre."
British authorities have previously admitted mistakes in their investigation into the schoolgirls, after their families complained police should have contacted them during an earlier investigation into the disappearance of a classmate.
Scotland Yard had met with Shamima Begum, Kadiza Sultana, and Amira Abase about the disappearance of a classmate with the girls handed letters to take home that asked their parents for permission to take statements, but the girls had hid the letters.