Privacy commissioner Jill Clayton announced Thursday that while Johnson was OK using the business emails of teachers, personal email is out of bounds.
"The heart of this issue is transparency" said Clayton in a news release.
"Even if you have authority to share information, it is important to be up front with people as to why their personal information is being collected and how it will used."
The office, among other recommendations, is urging Johnson's department to purge the personal emails from its system.
Johnson, in an email, said he will comply, but said there needs to be a middle ground.
"As the ministry is both the registrar and regulator of the teaching profession we will work with the privacy commissioner to ensure we develop a method of communicating on professional matters that reaches as many teachers as possible while also respecting the commissioner’s ruling," said Johnson.
"I believe teachers deserve to hear from me directly about issues and developments that may affect them."
Johnson said the department will continue to communicate with teachers through email.
He said many teachers aren’t provided with a work email account and have to use their personal email address in a professional capacity.
The investigation was launched after Johnson sent an email to more than 34,000 teachers in February.
Written in a conversational style and signed "Jeff," the email urged greater co-operation and exchange of ideas between the ministry and teachers on everything from curriculum design to contract talks with the Alberta Teachers' Association.
"We need to have open communications and ensure you have access to accurate information," said Johnson in the email.
"You will be hearing from me on a regular basis."
Clayton's investigation found that while the department can contact teachers to collect information for "evaluative analyses" on education, the email failed to do so.
"In fact, the minister does not refer to any purpose for which he is requesting that teachers contact him other than to state 'we need to have a more open and transparent way to communicate,'" said the report.
Mark Ramsankar, president of the Alberta Teachers' Association, said it was a just ruling.
"It very plainly states the ire that teachers felt and how really upset they were when the emails first came out," said Ramsankar.
"Communicating with teachers is one thing. Obtaining their personal email for direct access is something altogether different."
NDP critic David Eggen said the rebuke confirms that Johnson abused his authority, and is doing an end run around the ATA.
"(It) shows a lack of respect for the teachers and a lack of respect for the law," said Eggen.
Wildorse education critic Bruce McAllister called for Johnson to apologize to teachers.
“There’s a reason why this legislation is in place. It is not a recommendation, it’s the law,” said McAllister.
The privacy ruling is another thorn in what has become a prickly relationship between Johnson and the ATA.
In May, Ramsankar said task force recommendations on education reform now under consideration by Johnson constitute a "direct assault" on the profession.
The Task Force for Teaching Excellence report says the ATA is in a conflict of interest over the dual role of protecting teachers while at the same time being responsible for enforcing teacher discipline.
The report says the Education Department should handle allegations of teacher misconduct. It also recommends that school principals take over teacher evaluations with an eye to teachers effectively having to re-certify every five years.
Tensions went up another notch earlier this week, when Johnson ordered school boards across the province to deliver to him by July 11 documentation going back a decade relating to teacher discipline and complaints.
The ATA has asked Clayton's office to weigh in on this issue, too, saying a pell-mell race to deliver information to Johnson by next week could see private information accidentally spill into the public realm.