Southbrook School near Exeter, Honiton Community College and Mill Water School near Budleigh Salterton will close for the day due to the risk of ice, Devon County Council said.
Cornwall Council said Bugle school near St Austell will delay its opening until 10:00.
Great Western Railway has warned of disruption to its services due to the weather.
In a post on X, external, it said the line between Crediton and Okehampton was blocked.
It warned of cancellations to services between Exeter St Davids and Okehampton via Crediton.
Speaking to Radio Devon, Chris Lucas, 48, who travelled to Cornwall, said he joined the A30 at about 06:20 GMT on Thursday.
He said a snowy hill caused delays, adding he arrived home at about 18:15 GMT.
“People were so desperate to get home, they were trying to get up the hill and were getting stressed,” he said.