Chicago's third teachers' strike in three years ended today when negotiators for the 28,000-member teachers union and the city school board agreed on a new two-year pay and benefits package.

The walkout delayed by one day the opening of school for 429,000 pupils. At a news conference this afternoon, negotiators said classes will convene for a brief first day Thursday, with regular instruction beginning Friday.

The proposed new contract calls for a 6 percent pay increase this year and a 3 percent increase next year.

A systemwide referendum to ratify the contract will be scheduled in the next 10 days.

Gov. James R. Thompson, a Republican who has announced that he will seek his fourth term next year, intervened in the strike and took direct credit for settling the dispute by changing the payment schedule for state aid to Chicago's schools.

Using his executive powers, the governor ordered a two-year revision in the pace of statewide school funding, shifting a $65 million payment from the 1987 budget to the 1986 budget. Chicago's share of that money would be $23 million.

"This is no bail-out. There is no new state aid," Thompson said at the news conference.

Under the agreement, the 3 percent increase next year is conditional on the school board's success at raising the extra money from the legislature.

Extra aid for the city and its institutions is a perennial battle between rural and urban interests in the state legislature.

The school district has been wracked with deficits in recent years and plagued with a declining enrollment.

Meanwhile, several recent studies have detailed a sharp decline in students' performance in such basic academic skills as reading, writing and counting.

Today it was reported that the dropout rate is 43 percent, one of the highest in the nation, and that barely more than half the children graduating can read at better than an eighth-grade level.

Thompson said the accord means "stability for the kids, stability for the system and stability for the city." Pointing out that his daughter attends a city public school, Thompson said, "The system has been hanging by its fingernails for years."

The teachers had sought a 9 percent increase this year alone. The board's offer, rejected by the union, was 3.5 per cent.

A well-informed expert on the troubled Chicago system said the pay dispute is "a symptom of the bankruptcy of the system. As they approach the school year, teachers are like soldiers getting ready to go back into combat. Since nothing else changes, the one change they look for is more money."

Meanwhile, the Associated Press reported that teacher strikes idled a total of 151,000 students today in Seattle; Pawtucket and Newport, R.I.; Toronto, Ohio; four Michigan districts, including Pontiac and Flint; several in Pennsylvania and the Wheaton-Warrenville District in Illinois.

No new negotiations were scheduled in Seattle, where 44,000 were to have started classes today. The 3,700 teachers, aides and substitutes struck Tuesday over state-imposed salary limits and class size.

Pawtucket's 600 teachers voted early today not to work without a contract. They are seeking raises totaling 27 percent over three years and have rejected a one-year contract offer with a 5 percent raise. The strike affects 8,200 students.

In Newport, 330 teachers struck, idling 3,900 students.

The Pennsylvania strikes affected Butler County, the Pittsburgh and Greensburg Catholic dioceses, Montour and Peters Township