Driving difficult after snowfall
Fresh snowfall and sub-zero temperatures on Thursday night have led to further traffic disruption in the north west.
A severe weather warning is in place, and drivers are advised to drive with care, particularly on minor roads.
Met Eireann have warned that six inches of snow is expected in parts of County Donegal on Friday.
Colin Brown from Roads Service said crews have been salting the scheduled road network "all night".
"Care should be exercised on all our routes, particularly untreated minor roads."
In County Londonderry, Glendermott Primary and Nursery School in Ardmore, St Mary's PSl, Altinure, and St Canice's PS in Feeney are closed.
In County Tyrone Drumlegagh PS near Newtownstewart, Our Lady of Lourdes PS in Greencastle, St Patrick's PS in Gortin, Gortin PS, Killen PS and Bridge Hill PS in Castlederg and Castlederg High School are also shut.
There are reports of drifting snow in Park and Feeney in County Derry, and in County Donegal conditions are said to be particularly treacherous around Buncrana and Carndonagh.
At City of Derry airport, the 0915 GMT flight from Dublin has been cancelled. Passengers are advised to contact their airline.
The 0935 GMT departure to Dublin has also been cancelled, and passengers are to be transferred by road.