Schools must do more to protect female teachers following an "enormous growth" in the number of reports of upskirting, a teachers' union says.

The NASUWT union says it was aware of cases of upskirting - where pictures are taken without permission under a skirt - involving pupils aged 14, with "some as young as 11".

The union says head teachers should consider banning mobile phones in school and filling in open stairwells to protect both staff and pupils.

On 12 April, upskirting became a criminal offence in England and Wales.

This followed a campaign led by Gina Martin, who became a victim of upskirting at a music festival in London in 2017.

Offenders now face up to two years in prison for taking a photo or video under someone's clothing.

The NASUWT, which is meeting for its annual conference in Belfast over the Easter weekend, says often victims are unaware that the abuse has taken place.

"Talking to members about it, the thing they find the most difficult is that quite often they don't know that this has happened - the video has been out there and then it is drawn to their attention," said general secretary Chris Keates.

"Then they think, if I go and report it, is that going to make it worse because it will draw attention to the fact that the video is there."

Ms Keates said the union had seen "an enormous growth in the number of women contacting us".

"We haven't had a case of upskirting in primary schools - it's been secondary schools. We've had it in all age ranges. We've had some 14-year-olds and we've had some as young as 11."

She said banning mobile phones was the best way to protect staff, as well as pupils.

"Taking the mobile phones off pupils when they come into school is the best way to go because it ensures the health and safety and protection of everybody - pupils and teachers," she said.

Ms Keates said new school building designs should not include open stairwells, to protect privacy and dignity.

"It's just simple things, like when schools are being rebuilt, putting open stairs up, that kind of thing that people don't think about when they are doing these wonderful designs on buildings - things that can be an invasion of privacy.

"A lot of places now, even just in work places outside schools, are blocking those kind of stairs as well."

Earlier this year, the NASUWT supported two of its members in Northern Ireland who were the victims of upskirting by a pupil.

In February, an 18-year-old boy was found guilty of committing acts of outraging public decency, after he took five pictures of two female teachers at Enniskillen Royal Grammar School in 2015 and 2016 when he was 14 and 15.

Speaking about the case to union members gathered in Belfast, Ms Keates praised the "courage and determination" of the two women, saying they had done a "great service" not just for women teachers in Northern Ireland, but for women generally.

"I cannot begin to do justice here to the strength and courage of our members who have shown magnificent resolve at every stage of a long and difficult process.

"Upskirting is a serious assault. Upskirting is a vile and deplorable form of sexual harassment and objectification of women."

She added: "The NASUWT intends to use this victory as a basis to campaign for the necessary legislation here [in Northern Ireland] for protection not only from upskirting but also from all forms of image abuse."