From designing motivational posters to high-fiving students at the door, South Florida schools spend a lot of effort to get kids to do the obvious: Show up.

A new school year begins Monday with the reality that hundreds of thousands of Florida students — almost one in 10 — miss a month or more of school every year, according to state figures.

“Sometimes they’re absent for reasons that can be fixed,” said Laurel Thompson, Broward’s director of student services.

The reasons for chronic absenteeism, defined by Florida as 21 or more days missed per year, run the gamut from serious illness to avoiding a bully to just having trouble getting there.

A report issued earlier this year by Johns Hopkins University researchers found better attendance to be a painfully obvious — yet somehow routinely overlooked — way of boosting graduation rates, student achievement, and even standardized test scores.

“Being in school leads to succeeding in school,” the report states. “Achievement, especially in math, is very sensitive to attendance, and absence of even two weeks during one school year matters.”

The problem tends to snowball over time. Students can miss six months or even a year’s worth of school over five years, the study found.

Schools with the highest concentration of poor students tend to struggle the most with no-shows.

Chronic absenteeism hits the youngest and the oldest students the most: students in kindergarten through second grade and then rising again among middle and high school students.

Lyz Thompson, 24, works with a program called City Year at Miami Jackson Senior High in Allapattah — a school that is pushing the importance of simply showing up. Nearly 15 percent of Jackson students were chronically absent in the 2010-11 school year. But the school has made strides in its attendance, graduation and academics.

City Year, which is part of AmeriCorps and works with 15 Miami-Dade schools, focuses on the ABCs: attendance, behavior and course performance.

Why some skip

Last year, Thompson helped make daily calls to students who missed school. More often than not, the number on file would be wrong.

Why were these students missing class? One didn’t have an alarm clock. Others had to take care of their younger siblings. Some just didn’t seem to care.

“One kid would have to walk 30 blocks to school. His mom wouldn’t drive him or give money for the bus,” said Thompson, who’s returning to the school as the City Year team leader. They connected the student with a program to solve the problem. “He wanted to be here.”

At a recent orientation at Miramar’s Everglades High School, school officials stressed to parents the importance of regular attendance and how difficult it is for students to catch up on missed work, said parent Maggie Macaulay.

Macaulay was surprised to learn the percentage of Everglades students who are chronically absent — 22 percent.

“Almost a quarter,” Macaulay said. “I’m stunned ... and they’re an A school.”

Lots of other South Florida schools have higher percentages — much higher. About 30 percent of students are absent 21 or more days from Hollywood Hills High School; nearly 34 percent at Miami’s Jose de Diego Middle School.