Teach for a day, reap benefits for life.

Not satisfied with concessions extracted from the pension-reform legislation signed yesterday by Gov. Paterson, the state's powerful teachers union is urging would-be educators to exploit an outrageous loophole that lets them lock in sweet retirement benefits after just one day in the classroom.

New York State United Teachers posted a notice on its Web site Wednesday explaining how to game the system: Substitute-teach for one day before the new system starts on Jan. 1, apply for union membership, and you can join the more generous pension system any time in the next seven years.

"There's still a very small window — a matter of days — for the future members to lock-in Tier IV," the post reads.

The new Tier V pension system, which Paterson estimates will save taxpayers $35 billion over the next 30 years, boosts the minimum retirement age for public employees and increases the amount of time they have to put before becoming vested in the system.

The teachers unions, who have 600,000 members and led Albany special interests by spending $6 million in lobbying last year, extracted several concessions to make the new tier more favorable to educators.