Tomalis declined to be interviewed for this article. Corbett's chief spokesman, Kevin Harley, also would not comment.
The sources said a key to Tomalis' decision was growing behind-the-scenes tension between him and some members of the governor's inner circle. They would not elaborate.
As the person running one of the biggest departments in state government, with a $10.5 billion budget, Tomalis is one of the more visible members of the administration.
He has worked on a number of issues that have became political flashpoints, such as Corbett's 2011 push to implement tuition-voucher legislation. That was widely perceived as a fumbled effort, with many pro-voucher legislators complaining that Corbett did not come out strongly enough on the issue, which ended up withering on the legislative vine.
The issue set the administration - and Tomalis - on an early collision course with the state's largest teachers' union, the Pennsylvania State Education Association, which opposes vouchers. That relationship, especially after Corbett's cuts to public school funding in his first two years in office, has yet to be mended.
Tomalis also pushed for a 2012 law that changes how public-school teachers are evaluated and took strong steps after revelations of possible cheating on the state's standardized tests.
In 2011 he ordered forensic reviews of all exams since 2009, with special attention to Philadelphia. And he called for a probe of allegations of widespread cheating on Pennsylvania System of School Assessment tests. After he imposed unprecedented security measures on the 2012 exams, many scores dropped.
He also has cracked down on cyber charter schools, which his department oversees. Tomalis moved to revoke two such Philadelphia schools' charters - one for disregarding education needs of students and misspending tax dollars (the school agreed to close), the other for not meeting requirements for online instruction. That school is fighting to remain open.
Tomalis served from 1995 to 2001 as a top education aide to Republican Gov. Tom Ridge. From 2001 to 2004, he was in President George W. Bush's education department, where he managed implementation of the act popularly known as No Child Left Behind.
Contact Angela Couloumbis at 717-787-5934 or firstname.lastname@example.org.
Inquirer staff writer Martha Woodall contributed to this article.