It’s a tough lesson for millions of students just now arriving on campus: Even if you have a high school diploma, you may not be ready for college.

In fact, a new study calculates, one-third of American college students have to enroll in remedial classes. The bill to colleges and taxpayers for trying to bring them up to speed on material they were supposed to learn in high school comes to between $2.3 billion and $2.9 billion annually.

“That is a very large cost, but there is an additional cost and that’s the cost to the students,” said former Colorado Governor Roy Romer, chair of the group Strong American Schools, which is issuing the report “Diploma to Nowhere” on Monday. “These students come out of high school really misled. They think they’re prepared. They got a 3.0 and got through the curriculum they needed to get admitted, but they find what they learned wasn’t adequate.”

Christina Jeronimo was an ‘A’ student in high school English but was placed in a remedial course when she arrived at Long Beach Community College in California. The course was valuable in some ways but frustrating and time-consuming. Now, in her third year of community college, she had hoped to transfer to UCLA.

Like many college students, she wishes she’d been worked a little harder in high school.

“There’s a gap,” said Jeronimo, who hopes to study psychology. “The demands of the high school teachers aren’t as great as the demands for college. Sometimes they just baby us.”

The problem of colleges devoting huge amounts of time and money to remediation isn’t new, though its scale and cost has been difficult to measure. The latest report gives somewhat larger estimates than some previous studies, though it is not out of line with trends suggested in others, said Hunter Boylan, an expert at Appalachian State University in North Carolina, who was not connected with the report.

Analyzing federal data, the report estimates 43 percent of community college students require remediation, as do 29 percent of students at public four-year universities, with higher numbers in some places.