"We shall only release you after the struggle". You will be going to school now here, " the men said on the video.
The separatists have ordered curfews and closed down schools to protest President Paul Biya's French-speaking government.
A large majority of the kidnapped people are school children while the rest are the school staff, including the principal.
Dozens of people, a lot of them children, have been kidnapped from a school in the city of Bamenda in Cameroon's northwestern Anglophone region, which is struggling with a separatist insurgency.
The Associated Press reported a video was posted on social media claiming to show some of the kidnapped children.
Another source said that 79 of the kidnapped were children.
The government and English-speaking separatists have accused each other of orchestrating the kidnapping.
Hundreds have been killed in Cameroon's Anglophone regions in the past year, where violence between armed separatists and the military has increased since a government crackdown against protesters in the northwest and southwest regions. Cameroon is a country still deeply divided by colonial influences of English and French- some of the English minority support the creation of a separate English state in the West called Ambazonia, and in the previous year militias have formed in pursuit of that goal. One of the original gripes was that French-speaking teachers were being deployed to English-speaking schools in the Northwest and Southwest regions. Government forces have fought separatists in Cameroon's North West and South West regions.
More than 300,000 people have fled the violence, many of them living hand-to-mouth in the forests, and some across the border into Nigeria. The school is located near Bamenda, the capital of the troubled, English-speaking region.
The AP said it could not independently confirm that the video was made by the kidnappers.
President Paul Biya, who has been in power since 1982, was recently re-elected for a seventh term with more than 70% of the vote.
Earlier, a military source told Reuters that the school principal has been kidnapped along with the students.
About 20 percent of Cameroon's 22 million people are English speaking.
The school's headmaster, teacher and a driver were taken along with the children.