Tens of thousands of students in sixth form colleges across England are having their studies disrupted by strike action.
The National Education Union (NEU) says around 2,000 members are taking part in a walkout over pay.
In September, most teachers in schools and academies received a 5.5% pay reward, but sixth form colleges were not included in the pay uplift.
The Department for Education says sixth form colleges are responsible for setting their own pay.
Outside Holy Cross College, in Bury, Greater Manchester, around 20 teachers were protesting on Thursday morning.
"It's very cold, but we are willing to brave it," said Peter Barrow, the NEU reprensative at the college, which has more than 2,000 students.
"The pay disparity between sixth form colleges and schools threatens the sector as a whole," he said, adding he hopes the dispute will be resolved quickly.
Two more strikes days are planned next week, on 3 and 4 December.
Sixth form colleges are typically larger than sixth forms attached to schools, and can support thousands of 16-19 year old students studying for A-levels, as well as technical and vocational qualifications.
Many students are currently preparing for exams or GCSE resits, but colleges say they are putting measures in place to prevent major disruption.
Education Secretary Bridget Phillipson awarded teachers in primary and secondary schools an above-inflation pay deal in September, as recommended by the pay review body - with an additional GBP1.2bn given by the government to help fund the increase.
Teachers in sixth forms attached to schools, and those colleges that have converted to academies, have received the pay rise as they are funded directly government.
NEU members at 32 sixth form colleges , externalwhich are not academies, have decided to strike - impacting around 80,000 students.
Source: BBC