"I hate to say I told you so. But I told him so," she said on the CBC Radio's Metro Morning.
"So now, there's a lot of bad will, there's a lot of anxiety, and the people of course that are getting the brunt of it are the students and the parents," she said.
Earlier this month, the Ontario government imposed two-year contracts on its teachers after being unable to negotiate a contract. Many teachers had stopped performing voluntary work, such as supervising extracurricular activities and coaching school sports teams.
When pressed for ideas on how clean up the mess, Horwath didn't offer any. Instead, she said it will be up to whoever replaces McGuinty as premier.
"It's going to be up to the new premier to mend relations," she said.
"If it was me, it would be a different story. If I was in the premier's chair, first of all, we wouldn't be here. Second of all, I wouldn't have the history that the Liberals have put in place in terms of their relationship.
"They went into a situation claiming that they wanted to have a conversation, and instead put out an ultimatum," she said.
Horwath said the relationship between the government and the teachers has been "damaged significantly."
"There's a real trust that has been broken," she said.
Horwath praised the province's relationship with its teachers over a 10-year period.
"And they threw it all away, just for their own political interests, which I think is the most galling thing of all.
"It wasn't for stability in the school system, or to make education better. It was to win a byelection in Kitchener-Waterloo, which of course they lost," she said.
In that byelection, last September, the NDP candidate, Catherine Fife, won, while the Liberal, Eric Davis, finished third.
Horwath was also asked if the teachers' decision to withdraw extracurricular activities should be reconsidered. But she said that should be up to the teachers to decide.
"I leave it to them to decide how much ill will they're going to allow to fester," she said.
When it was pointed out to her that Conservative Leader Tim Hudak has made public specifics policies on several issues, but she has not, Horwath said, "I work quite differently."
She said her party has spent the last several months in consultation with Ontario families, professionals and the business community about issues such as jobs, unemployment, health care, and "the affordability of everyday life."
Horwath said she is not in an election campaign.
"When we're in an election campaign, there's no doubt that there's going to be some solid policies that we're putting forward," she said.
"People will be very clear about where the NDP is on all of the various Issues."Suggest a correction