Preparing for her daughter’s graduation in the spring, Tuli Chediak received a blunt message from her daughter’s charter high school: Pay us $600 or your daughter won’t graduate.

She also received a harsh lesson about charter schools: Sometimes they play by their own rules.

During the past 15 years, Florida has embarked on a dramatic shift in public education, steering billions in taxpayer dollars from traditional school districts to independently run charter schools. What started as an educational movement has turned into one of the region’s fastest-growing industries, backed by real-estate developers and promoted by politicians.

But while charter schools have grown into a $400-million-a-year business in South Florida, receiving about $6,000 in taxpayer dollars for every student enrolled, they continue to operate with little public oversight. Even when charter schools have been caught violating state laws, school districts have few tools to demand compliance.

Charter schools have become a parallel school system unto themselves, a system controlled largely by for-profit management companies and private landlords — one and the same, in many cases — and rife with insider deals and potential conflicts of interest.

In many instances, the educational mission of the school clashes with the profit-making mission of the management company, a Miami Herald examination of South Florida’s charter school industry has found. Consider:

• Some schools have ceded almost total control of their staff and finances to for-profit management companies that decide how the schools’ money is spent. The Life Skills Center of Miami-Dade County, for example, pays 97 percent of its income to a management company as a “continuing fee.” And when the governing board of two affiliated schools in Hollywood tried to eject its managers, the company refused to turn over school money it held — and threatened to press criminal charges against any school officials who attempted to access the money.

• Many management companies also control the land and buildings used by the schools — sometimes collecting more than 25 percent of a school’s revenue in lease payments, in addition to management fees. The owners of Academica, the state’s largest charter school operator, collect almost $19 million a year in lease payments on school properties they control in Miami-Dade and Broward counties, audit and property records show.

• Charter schools often rely on loans from management companies or other insiders to stay afloat, making charter school governing boards beholden to the managers they oversee. Loans to two Pompano Beach schools were disguised as gifts in financial documents to avoid scrutiny from the school district and make struggling schools appear solvent, the schools’ former managers said in court papers.

• At some financially weak schools, tight budgets have forced administrators to cut corners. The cash-strapped Balere Language Academy in South Miami Heights taught its seventh-grade students in a toolshed, records show. The Academy of Arts & Minds in Coconut Grove went weeks without textbooks. Schools have also been accused of using illegal tactics to bring in more money — charging students illegal fees for standard classes, or faking attendance records to earn more tax dollars, court records show.