Union spokesman Carlos Ramirez says drivers will begin their strike in 11 cities on Monday and continue indefinitely.
Island officials say a $2 billion deficit has made it hard to pay millions of dollars owed to employees and contractors who work in several sectors, including education.
The government has not yet responded to union threats.
Ramirez on Saturday said some 400 unionized bus drivers ferry 60 percent of the island's estimated 610,000 students to class every day.
School is scheduled to continue through mid-December.