In his inaugural address earlier this month, Mayor Marty Walsh brought up, once again, an issue he's been pushing for since he ran for mayor in 2013: universal pre-kindergarten. 

"We will keep working with the Legislature on our plan to fund universal pre-kindergarten with tourism taxes that are already being collected in Boston," Walsh promised.

There's a reason he keeps bringing it up: studies overwhelmingly show that students who attend preschool are better prepared for kindergarten than ones who don't, and poor and disadvantaged children benefit even more.

Yet the salaries early education teachers receive do not reflect how important their role is, says a recent report in The New York Times titled "Why Are Our Most Important Teachers Paid The Least?" The average pay for an early childhood educator is $28,500, according to The Times, while the average salary of a K-12 teacher is $53,100.

Harvard Graduate School of Education professor and former state Secretary of Education Paul Reville said the pay gap was a huge roadblock to expanding increased access. 

"The differentials are huge," Reville said.

"As a result, you have a market for teachers in preschool that's constantly churning, where you're attracting a lot of people without necessarily deep qualifications to do the work, and they can't sustain themselves there, or — given the nature of the work, which is 52 weeks a year, eight hours a day, usually in these situations — they don't have a lot of time to get advanced degrees," he continued.

Reville explained that low pay means that when early childhood educators do achieve higher degrees, they quickly move to teaching other levels because there is little incentive to remain where they are.

"Typically, our experience has been, when states or others have tried to provide resources or funding available to upgrade the quality of teaching in early childhood ... those people — for example, those who have gotten B.A. degrees — are frequently siphoned off by the K-12 system, where they can make double the money that they're making in early childhood," Reville said. "We've got to figure out how to upgrade the quality of teaching while, at the same time, expanding access to early childhood."

Click the audio player above to hear more from Paul Reville.