Will Detroit's public schools open Tuesday?

(CNN)When Detroit's parents wake up Tuesday, it's not clear whether they'll be able to send their kids to school.

All but three of Detroit's 97 public schools were closed on Monday after more than 1,500 teachers called out sick in protest, school district spokeswoman Michelle A. Zdrodowski said.

There are more than 3,000 teachers in the district, according to state documents.

The union which represents many of them, the Detroit Federation of Teachers, told its members they don't expect them to show up for work Tuesday either. (Detroit parents -- the public school system said on its Facebook page that they usually can confirm if school will open between 5 a.m. to 7 a.m.)

Here's why.

Detroit's Public School system is deep in the red -- they have more than half a billion dollars of operating debt, the governor's office has said.

Detroit teachers were told by DFT Interim President Ivy Bailey on Saturday that the school system only has enough money to pay its teachers through June 30. Some school employees can elect to have their pay spread out over the entire year rather than the school year, which means that if they don't get paid after June 30, they've already started working for free, DFT argues.

The average teacher salary in the district is $63,716.

"There's a basic agreement in America: When you put in a day's work, you'll receive a day's pay. DPS (Detroit Public Schools) is breaking that deal," Bailey said.

(Incidentally, this week is National Teacher Appreciation Week -- an observance the Detroit Public School system is touting on their website's homepage.)

The union also told CNN it will run an advertisement in the Detroit Free Press on Tuesday with a similar sentiment.

"It's the law that when you work, you get paid," the ad says. "Asking teachers to work without pay is un-American."

Teacher salaries are the latest casualty of the crisis -- with so much money going to pay down debt, many of the school system's facilities have fallen into disrepair, teachers say.Teacher strikes are illegal in Michigan. So to protest the poor conditions that educators say they and students must deal with, Detroit Public School teachers have relied on the "sickout" method -- calling out sick en-masse, forcing schools to close.

In January, teachers staged a sickout to protest dilapidated and dangerously unsanitary conditions -- including rat and roach infestations, black mold and falling ceiling panels -- forcing the closure of dozens of schools. A judge later ruled that teachers could continue staging the sickouts after DPS brought the union to court over the issue.

That's why they are using sickouts to protest the news about their pay.

More than 500 people attended a rally in support of the city's teachers Monday morning outside a school district administration building, according to Nikhol Atkins, a staff member at the teachers' union."I support the teachers on getting a fair deal. They're educators," Detroit parent Tony Kinsey, whose sons attend 11th and 9th grade, said. "I'm frustrated with the adults, the leadership. Our children are the ones suffering."Teachers and some parents are urging Michigan lawmakers to pass a $715 million education reform package that would fund salaries for July and beyond.

The legislation has passed the Michigan Senate, but still needs to be approved by the House and Gov. Rick Snyder.

"I have been and remain confident that the Michigan legislature understands the urgency and importance of the reform legislation that is before it," Judge Steven Rhodes, who was appointed in February by Gov. Snyder to be the transition manager for Detroit Public Schools, added Monday. "The future of Detroit is as much at stake here as the future of the school system."

The DFT said its leadership met with Rhodes and other officials from the school system Monday, hoping to get assurances that teachers will be paid for their work.

"They refuse[d] to say the three words our members need to hear: 'I guarantee it,'" the union said on its website. "Their failure to give us that guarantee is tantamount to a lock-out. And since we have not gotten the guarantee that members will be paid."

Rhodes called the sickouts "drastic" and "unnecessary," but said he was sympathetic to the teachers' plight.

"I am on record as saying that I cannot in good conscience ask anyone to work without pay," said Rhodes. "Wages that are owed to teachers should be paid ... I understand the frustration and anger that our teachers feel."Sharlonda Buckman, CEO of Detroit Parents Network, an organization of parents with children in all city schools, said she felt an "instant splitting headache" when she heard about the sickout.

"This is one of the most tumultuous school years our kids have experienced," she told CNN. "They aren't getting what they need. It's disturbing. First in January ... (now) we're in May and this is still happening."

Buckman has nieces and nephews in the public school system. She points out that not every parent has the flexibility to stay home from work or to be late when faced with closed schools, and for some that means children are babysat by older children or left entirely alone.

"This creates a safety issue when you have unsupervised children," she said.

It's also a challenge for parents like Kinsey, who works from home but has had to interrupt work three or four times to cajole his sons to do schoolwork. His 11th grader needs to prepare for the SAT, he said.

"I gave them a couple choices, as long as it was learning," said Kinsey, who gave his sons reading assignments. "They think this is a vacation. My oldest wants to go to the movies and the mall. It's been a lot of negotiating, going back and forth and empathizing with them. It's been tough."