At least two school districts have set deadlines for parents to vaccinate their children or risk their exclusion from the classroom.

The Colorado Sun reports that Littleton Public Schools and the Boulder Valley School District are enforcing state law, which requires students to have either current immunization records or a personal, medical or religious exemption form on file.

Boulder Valley has the state’s highest exemption rate, with approximately 1,100 out of its 30,000 students without documented immunization, The Sun reported.

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“Students who do not submit an up-to-date certificate of immunization or a written letter confirming the date of appointment for required immunization(s) or a valid exemption will be suspended and/or expelled from school according to this policy’s accompanying regulation,” the Littleton school board’s policy reads.

In the two school districts – as well as 27J in Brighton last year – the majority of parents whose students were unvaccinated chose to get the vaccine rather than exempt their child, according to The Sun report. The response suggests that parents were unaware of, rather than opposed to, vaccination schedules.

The state cannot penalize schools who fail to keep track of immunizations.

Colorado has the lowest vaccination rate of any state for two vaccines: the measles, mumps, and rubella, as well as the varicella (chickenpox). It is one of only three states where the coverage rate for kindergarten students dips below 90%.

Twenty-four thousand children in Colorado began school with non-medical exemptions this year.

This past legislative session, House Bill 19-1312 would have brought school districts into compliance with state rules on immunization, and would have created a process for seeking exemptions.

Gov. Jared Polis opposed it as written, citing the burden of submitting exemption paperwork to health departments.

“The minute you try to have the government forcing anybody to do something with their kids, you’re going to create distrust of vaccinations, which is already a problem,” Polis said at the time.

In June, Polis signed an executive order to boost education about immunization and study “vaccine hesitancy.”

Rep. Kyle Mullica, D-Northglenn, was one of the failed bill’s sponsors. He said that individual school districts’ decisions enforce the law would not solve the whole problem.

“Colorado has one of the easiest processes for opting out of vaccination, and we see schools with non-medical exemption rates of 30% or even higher,” he said. The measles vaccination rates in schools shows that almost 24,000 kids would be excluded in the event of an outbreak due to claiming a non-medical exemption.”

He added that while it is good to turn in immunization paperwork on time, he would like to see the state look to other solutions, ranging from limiting exemptions to requiring parental affadavits to greater education about vaccines. He is still working to identify legislative options.