After weathering weeks of criticism from the teachers union, Mayor Bloomberg is switching to offense in the battle over his plan to overhaul the schools.

Yesterday, Department of Education officials announced a new automated system that will encourage principals to crack down on teacher tenure. Just earlier, the mayor had lambasted the teachers union during a press conference. Suggesting that United Federation of Teachers officials should return to the classroom if they want the city to hire more teachers, Mr. Bloomberg warned that “union agendas” are jeopardizing his plans for the extra education funding expected to be allocated to the city in the state budget.

“If people want more teachers in the classroom,” Mr. Bloomberg said, they should look to “the union reps that … just sit around and get paid by the taxpayers and not teach.”

The United Federation of Teachers has lobbied state legislators to require that some of the new education funds go toward lowering class size, which would mean hiring more teachers. The Bloomberg administration has resisted the union’s calls to tie the money to class-size reduction.

“What worries us a lot more is … using the financial budget to push individual union agendas and some ideological agendas when it comes to education,” he said, responding to a question about the budget negotiations going on in Albany this week. “That is phenomenally dangerous, and hopefully that will not happen.”

The president of the United Federation of Teachers, Randi Weingarten, who was expected to win by an overwhelming margin in the union’s election yesterday, responded to the attack by saying: “If believing that helping kids by getting them the best teachers possible and reducing class sizes is ideological, then call me an ideologue on behalf of children.”

The new tenure notification system will automatically send principals monthly e-mails to inform them of teachers becoming eligible for tenure, allowing them to either approve or reject the teachers online.

“Not everybody is going to be a good teacher,” Chancellor Joel Klein said. “We cannot afford to let ineffective teachers remain in our system.”

The main difference between the new process and the old is the shift from paper to an automated system. Principals now are able to review a teacher’s status online, but notifications and forms about teachers are sent to principals in the mail and must be returned in the mail.

Ms. Weingarten said the system introduced yesterday is not new, but re-creates a process used by the old school boards, which were dismantled by the mayor.