They've tried mass mailings, personal letters, telephone calls and "robocalls" to homes.
They've run notices on their websites, sent home fliers with report cards and sponsored clinics.
But apart from sending out certified letters, school officials have had little success trying to coax, cajole or otherwise convince parents to get their children immunized so they can attend school.
And the clock is ticking.
The state Department of Health issued new immunization requirements in August, but it allowed students who lacked the shots to attend school on a provisional basis for eight months.
That leeway expires May 1, when all schools will be required to start barring students who lack the immunizations - and records that they received them - from entering school.
If the ban were to be imposed today, more than 5,500 public school students in Lancaster County would be sent home. And there are probably hundreds more pupils attending private schools facing the same fate.
School officials are concerned that parents will wait until the last minute then flood doctors' offices seeking the shots, creating a backlog that could keep their children out of school for days at a time.
That could result in parents being cited for truancy.
"We, sadly, once again look like the big bad district harming the children, yet it's the state's rule and the parents' obligation," Ruth Crawford, director of student services for Hempfield School District, said in an email.
About 600 of Hempfield's roughly 7,000 students are not in compliance with the regulations.
School District of Lancaster has about 1,600 students without proper vaccination records - nearly 15 percent of its entire population, despite hosting clinics at its schools and conducting multiple mailings and telephone notifications.
In Columbia, nearly a quarter of all students - a total of 347 - lack documentation that they've been immunized.
The percentages tend to be lower in wealthier districts, where children are more likely to visit a doctor regularly. But all 17 county districts report having students who have yet to comply with the regulations.
"It's frustrating," said Bob Hollister, superintendent of Eastern Lancaster County School District, where about 150 students lack updated records. "I don't know what to speculate is the issue unless it's money and people just can't afford another visit to the doctor."
Pupils in all grades this year are required to get a second dose of chickenpox vaccine if they haven't had the disease. All seventh-graders need one dose of the Tdap vaccine against whooping cough and a dose of meningococcal conjugate vaccine (MCV).
Vaccine supplies are plentiful, according to local pediatricians, who said they haven't seen an increase in demand at their offices recently.
The shots have been recommended for years by the Centers for Disease Control and Prevention, so children who regularly see a doctor likely have gotten them already, said Dr. Jennifer Ammons of Roseville Pediatrics.
It's the children who rarely go to a doctor who are running afoul of the regulations. And that includes many older children, who have fewer regular checkups than their younger peers.
In Manheim Township, for example, 50 high school seniors are among the 433 students who have yet to get all their shots.
Like other local districts, Township sent letters home, telephoned parents and distributed notices on the immunization requirements, and the high school principal spoke individually with students who have yet to get the shots.
"We don't know what else to be doing," said Kathy Setlock, the district's director of pupil services. "How much hand-holding can you do?"
Solanco has been more successful than most districts in getting parents to comply. Only seven of its students lacked the shots this week, down from more than 150 at the start of the year.
Before the school year started, the district required that parents of students without the vaccines sign an agreement to get the shots before their children could attend school.
Those who still didn't comply got a certified letter informing them of the mandate. The district also offered the vaccines at its schools through a "catch-up" program provided by the Department of Health.
Officials at larger districts say they lack the resources - the certified letters cost $5.59 apiece - to mount such an aggressive outreach.
They don't want to bar students from their schools, school officials said, but if the pupils don't have records of being immunized, they'll be forced to keep them out.
"I know for a fact that we have students with compromised immune systems in our schools," Setlock said.
"How can I say I'm not going to enforce this when it's putting them at risk?"