The first skirmish of the Boston mayoral campaign broke out yesterday after challenger John Connolly told the Herald that Mayor Thomas M. Menino deserved an “F” grade on improving the city’s schools and Menino’s camp fired back at Connolly’s “politically charged” comments.

The sniping ended a short-lived truce in which the longtime mayor and upstart city councilor, who announced his mayoral bid this week, heaped praise on each other and promised to avoid a negative campaign.

But in the Herald’s High Noon web-cast yesterday, Connolly sharpened his tone against the incumbent, faulting Menino for caving to the teachers unions and failing Boston schoolchildren.

“I give him an A for effort but the grade, the one that ultimately counts, is a failing grade,” Connolly told the Herald in an extensive interview in the live webcast on bostonherald.com.

Menino spokeswoman Dot Joyce later accused Connolly of making unfair charges against a school system that has made major improvements in test scores and dropout rates.

“It is easy to pick on a district that has 60,000 students speaking 170 different languages with a variety of challenges and talents,” Joyce said, adding that Connolly was sitting on the sidelines while the mayor was working to negotiate a union contract giving principals more power to get rid of poor teachers.

“It is easy to say you’re for change, it is quite another thing to actually fight for it,” she said.

Connolly is the first candidate to challenge the 70-year-old Menino, who has battled a series of serious health problems and still not formally announced whether he will seek a record sixth term.

The 39-year-old Roslindale native faces long odds at toppling Menino, and denied he got any advance word that the mayor has decided to retire.

“I’m not privileged to any inside information,” he said. “He’s the only person who knows right now what he’s going to do. I’m prepared to run a campaign if he’s in or if he’s out and to run a campaign to win.”

Connolly also hinted he would make a City Hall housecleaning if he’s elected, including possibly replacing police Commissioner Edward F. Davis.

“I’m not going to say that right now,” Connolly said about firing Davis. “I will say it is a major goal of mine to bring a lot of new talent into City Hall.”

The interview with Connolly is available for viewing on bostonherald.com. Herald columnist Howie Carr will appear on today’s edition of High Noon.