Ten Glasgow schools were closed yesterday, with several others disrupted, over a row about staff having to take on extra duties involving healthcare and administering medicine.
Unison said workers were not being properly trained in the tasks or paid enough to reflect the responsibilities.
There will be another day of strike action on Wednesday next week, affecting schools in the north east of the city.
Hundreds of people demonstrated outside the City Chambers in George Square yesterday.
Seven schools for pupils with complex learning needs were shut - Broomlea School, Milton Secondary, Croftcroighn Primary, Newhills Secondary, Langlands Primary, Linburn Secondary and Hampden Primary.
Three schools for youngsters with physical, visual and hearing impairment - Ashcraig Secondary, Kelbourne Park Primary and Hazelwood School - were also shut.
Hyndland Primary was the only main-stream school which had to ask pupils in P1 and P3 to stay at home.
There are around 1500 support assistants at the schools.
Carol Ball, education convener for the Glasgow City Branch of Unison, said: "We are feeling positive that so many of our members came out for the day of action. They did not take this decision lightly but it shows how strongly they feel about taking on medical tasks that they have received no training for."
Councillor Stephen Curran said: "We are really disappointed and sorry that Unison has decided to go ahead with the industrial action which stopped our most vulnerable young people coming to school."