The Boston Public Schools have handed out millions in no-bid contracts in the form of nonprofit grants and consulting gigs for retired principals in a system that lacks accountability and oversight, a Herald review found.

School officials have signed more than 50 no-bid contracts so far this year in amounts ranging from $27,000 to more than $7 million for everything from a diploma program for at-risk students, teacher training seminars and art teachers to computer software updates, school bus insurance, and coordinators to lead “healthy play” at recess.

“I don’t know what their guidelines and measurements are on these contracts. I think sometimes they get into the system and then the contract is just renewed because they’re a known entity,” said Matt Cahill of the Boston Finance Commission, the city’s watchdog agency. “The school department likes to give principals a certain amount of auto-nomy. Unfortunately with more than 130 principals writing contracts, you get varying levels of success in the schools.”

State law requires governments to hold a public bid process for any contract worth $25,000 or more, but school officials often use loopholes in the law for nonprofit grants and job-related training.

A Herald review found:

• One headmaster who gave a retired teacher — now serving as a consultant at her former school — a plum contract this year that would have doubled her pay from $25 to $50 per hour until the Boston Finance Commission questioned the reason for the raise. Under pressure, the contract was later revised to her previous hourly rate;

• Varied pay rates, including $60 per hour for nine retired principals and two retired assistant superintendents to serve as mentors to new principals, while another retired principal signed a contract that earns him more than $71 per hour this year;

• A local nonprofit allowed to run a 10-week summer program staffed with graduate student interns before the group had officially signed a contract — an administrative failure that the Boston Finance Commission noted posed a liability to the school district.

Acting Superintendent of Schools John McDonough acknowledged problems with the summer program contract, which he said was delayed to due a disagreement over the nonprofit’s legal requirements, and he told the Herald the retired principal earning a higher rate was hired for administrative duties — not as a mentor.

But McDonough denied there is no oversight, telling the Herald all contracts are reviewed by lawyers, auditors, administrators and by him personally.

“There is rigorous review and even more rigorous review for any contract we are exercising in exemption for advertisement for public bid,” McDonough said. “We do not simply choose anyone who signs up and says, ‘We want to go to work.’ There are certain requests for contracts that don’t reach the Finance Commission. We screen them out.”

Cahill said McDonough has worked to add some guidelines — such as paying retired principal mentors a set rate of $60 per hour — but problems still exist.

“John McDonough, from a business perspective, is a brilliant guy, but I don’t think there’s enough people in there running around making sure each contractor is doing the right thing,” Cahill said. “If you can give the principals guidelines and say this is how much this person should get paid, and this is what the contract should look like, then you’re making their jobs easier. Anytime you can make a principal’s job easier, you should do it.”

BPS — the Hub’s largest department with half of the city’s workforce — controls a $937 million budget, or 36 percent of the city’s budget, according to the Boston Municipal Research Bureau. School officials are expecting costs to increase by as much as $60 million next year in large part due to employee raises, health insurance and a loss of federal funding — a shortfall that will face Mayor-elect Martin J. Walsh next month when he takes the reins of the city.

McDonough said, “I think ultimately the advice to the new mayor is, we do need to find the appropriate balance to recognizing school-based flexibility and decision-making that is supported by the central office, but is also credible and defensible.”