As they prepared to return to classes next week, Ontario's college students worried Thursday their studies could be jeopardized after unionized support staff at the schools walked off the job.

The Ontario College Student Alliance, an organization that represents students at 15 colleges, sounded the alarm hours after the Ontario Public Service Employees Union called a strike.

The strike "puts the availability and quality of support services in jeopardy for all Ontario college students at a time when they are most needed," president Brian Constantini said in a statement Thursday.

The group vowed to keep tabs on the labour stoppage in the coming days and urged the colleges and workers to reach a settlement before the fall semester starts on Tuesday.

The colleges have pledged to stay open during the strike, with management and non-unionized part-time staff stepping in to handle the crush of students slated to return to campus.

But the union said it's skeptical the schools will function smoothly without the striking workers.

"They haven't felt the full brunt of the students yet," said Rod Bemister, chairman of the OPSEU bargaining committee.

"Right now, there's not a lot of traffic, not a lot of students, but come Tuesday morning, it's probably the first time they'll feel it full force and see what support staff actually mean in the colleges."

The union represents about 8,000 cleaners, food service staff, registration officers and other workers at the province's 24 colleges.

Members rallied in front of George Brown College's St. James campus in downtown Toronto, waving signs and chanting as a few motorists honked in support.

"It's the first time I've been on strike," said Marilou Martin, a job placement co-ordinator who has worked at George Brown for 25 years.

"My job has always been to serve students, so not being able to do that is of course difficult," but it's important that workers stand up for themselves, she said.

Despite their concerns, some students are backing the striking workers, said Daniel MacDonald, spokesman for George Brown's student association.

"A big reason is because a large percentage of George Brown support workers specifically are George Brown alumni," he said.

It's in the students' interest to ensure good working conditions for their future jobs, he said.

The colleges said they offered increases for support staff that would take the average salary to more than $59,000 a year with no concessions.

"We have a good offer on the table," Don Sinclair, a member of the colleges' bargaining team, said in an interview.

"It's unfortunate that OPSEU has called this strike and it's unfortunate for our students, but I think OPSEU should really focus on what is best for students and return to the bargaining table."

The colleges' offer would see union members receive a 1.5 per cent salary increase for each of the first two years and 1.75 per cent in the third year.

The union has been seeking a three per cent annual wage increase and job protection against the schools' use of part-time and temporary hires.

The union said it went on strike because the colleges demanded concessions and clawbacks to its existing contract.

"They want to take more full-time jobs and turn them into more part-time jobs," the union's president, Warren (Smokey) Thomas told picketing workers at George Brown.

The union said part-time staff already outnumber full-time workers. At George Brown, there are about 560 full-time support staff, compared with roughly 860 part-time, it said.

Union members voted last month to give negotiators the option to call a strike.

The last time the support workers went on strike was in 1979. Workers approved a strike mandate in 2008 contract talks, which concluded without any work stoppages.