OSWEGO, Ill. (CBS) — Parents in one suburban school district are wondering what the days to come will bring after a bus driver shortage caused some schools to cancel in-person learning on Tuesday.

As CBS 2’s Suzanne Le Mignot reported, COVID-19 vaccinations may have something to do with all this.

READ MORE: Another Victim Of Serial Killer John Wayne Gacy Identified: Francis Wayne Alexander, Of North Carolina

A total of 24 school bus drivers and 15 bus monitors from the Oswego Transportation Association called in sick on Tuesday. Sunday was the start of mandatory proof of COVID-19 vaccinations or a negative test for employees.

The bus company serves 13 elementary schools in Oswego Community Unit School District 308.

Parent Christy Forbes has three children in school in the Oswego district. One in elementary school, another in junior high, and another in high school. While her son attended school in person Tuesday at Boulder Hill Elementary, her two other children did e-learning.

“They should have let the high schoolers and junior high kids, whose parents were able to drive them to school, let them go to school – and if they couldn’t, then let them e-learn,” Forbes said.

District director of communications and public relations Theresa Komitas said calls about bus drivers calling in sick started coming in as early as 4:30 a.m.

Elementary school students were still able to learn in person Tuesday with some bus schedule shuffling.

“We did have enough drivers to run those routes, though we kind of had to stack the routes,” Komitas said. “Some of them had to run a route, drop students off, and get another route and come back again, so we did have students arriving up to an hour and 15 minutes late today.”

While elementary school students were learning in person, nearly 9,800 junior high, high school, alternative program students had to learn remotely.

READ MORE: Teen Charged In Pair Of September 2020 Carjackings On South Side

The sick calls coincided with a mandate on Sunday that employees show proof of a COVID-19 vaccination or a negative test.

“If it is related, certainly the timing is, you know, is coincidental, although we can’t say we’re connecting the two together,” Komitas said.

Staff were told for the past several weeks that they would have to show proof of vaccination or a negative test result starting this week in order to work.

“They should have proof that they have the test, and they should get the shot,” said Oswego school grandparent Thomas Siebert.

“Unfortunately, what happened this morning, it puts a lot of parents in a hurry to situate themselves in how they’re going to get their kids to school or how they’re going to settle them at home,” said Oswego school grandparent Maria Mora.

“I would prefer to be back in the building, seeing my peers, and being involved with my teachers, but today was OK,” said Oswego High School student Evet Mora, adding that she hopes to be back in school on Wednesday.

The union representing the bus drivers had no comment for this story.

MORE NEWS: Desperate To Find More Staff, Some Fast Food Restaurants Recruiting Customers

We also asked what Wednesday might hold, and whether there will be sick calls again? The school district does not know, and the only option is to wait and see. But the district said there is a plan in place when that happens.

Suzanne Le Mignot