Teachers, firefighters and council workers will strike on the same day.
The National Union of Teachers (NUT), Fire Brigades Union (FBU) and unions that represent public sector workers will take industrial action tomorrow (Thursday July 10).
West Sussex County Council (WSCC) said services would be disrupted and warned parents to prepare for school closures.
A spokesman said: “Parents are being advised to check with their child’s school to see what impact the action is likely to have and whether the school will be closed or partially closed to some classes.
“Parents should be aware in some cases a decision to close a school may be made at very short notice.
“WSCC is continuing to work with unions to ensure that critical frontline services can be maintained for residents, although it is inevitable there will be some disruption to services on the day.”
Industrial action by Unison, GMB and Unite over workers’ pay will lead to disruption of other council services.
A spokesman for Crawley Borough Council said: “We won’t know how many staff will strike until next Thursday because they do not have to give advance notice of their intention to strike.
“However, we expect the majority of services will be able to provide a normal service.”
The FBU strike will last between 10am and 7pm.
The union disputes government plans for firefighters’ pay and pensions. The FBU planned further strikes from July 14 to 21.
Matt Wrack, FBU general secretary, said: “The government is merely ploughing ahead, forcing firefighters to react.”
The NUT’s industrial action is part of ongoing disputes over central government proposals workload, pensions and performance-related pay. Christine Blower, NUT General Secretary, said: “The Government is still failing to make progress on our trade dispute over teachers’ pay, pensions and workload.
“For teachers, performance related pay, working until 68 for a full pension and heavy workload for 60 hours a week, is unsustainable.
“Strike action is a last resort for teachers and we deeply regret the disruption it causes parents and pupils. This date has been chosen to cause minimum disruption to examinations.”