Image caption "Why didn't my child get all As?" is the type of question Lotta Juvin doesn't want to see in her emails

An overload of emails, including in the evenings and weekends, is threatening to leave England's teachers exhausted, with no off-switch from their working lives, warns the Education Secretary, Damian Hinds.

Speaking at the annual Bett Show for educational technology, he warned that feedback to parents had gone from a quick chat at parents' evenings to being expected to be available every hour of the waking day.

"I'm sure none of us now could imagine a life without email but do we ever stop to think how much of our day is actually spent reading or replying to them?" he said, in a speech at London's Excel Centre.

But what was the experience of teachers at the show?

Image caption Neelam Parmar says her school discourages sending or replying to emails in the evening

It's an event, with 34,000 visitors and 700 high-tech companies showing their devices, apps and software - and selling the positive impact of technology.

It's the type of place that can offer "solution dens" without even a trace of irony.

Jon Barker, head of digital at Highams Park School, in Walthamstow, east London, says schools have gone from being virtually an email-free zone to having a sudden surge of online communication.

"Everyone thinks that the email they're sending is important," he says.

"But a teacher's job is to teach a class, not to communicate with 30 sets of parents."

Image caption Jon Barker says that schools have to find ways to manage expectations about answering emails

The difficulty with emails for teachers, he says, is that they rapidly proliferate and every answer to a parent can start another set of conversations.

He says it's not "healthy" for staff to end up working late every night, trying to cope with answering complicated individual emails from dozens of parents.

And that's on top of all the internal admin emails that take up so much time in every organisation.

Neelam Parmar, director of e-learning at Ashford School, in Kent, says her school has a policy that discourages the sending of emails in the evening.

She says this is usually "respected by parents and teachers".

Image caption Allen Tsui says schools need to make sure there is a purpose to any emails sent out

"It's so easy for parents to want instant feedback," she says, so schools have to actively prevent staff being swamped by emails out of office hours.

The type of emails that teachers might receive, about specific problems with children, are also going to need more than a quick reply.

"Why did my child get a detention?" could take a bit of answering - and she says face-to-face meetings can be much better for talking to families.

It's not an insignificant amount of time.

Image caption Carl Ward says schools might prefer not to give out the email addresses of individual teachers

Laura McInerney, a former teacher and education journalist, gathers research data through the Teacher Tapp app, which suggests teachers are spending six hours a week answering emails.

Apart from the loss of time, how much creative energy disappears into multiple, quick-fire replies and keeping down the numbers of unanswered emails?

Allen Tsui, academic enrichment programme leader at Willow Brook primary academy, in east London, says schools are recognising the importance of preventing an excess of emails.

Image caption There are about 700 exhibitors showing their educational technology at the annual Bett Show

"Emails should have a purpose," he says. And that shouldn't be because people "need to be seen" by sending everyone an email on Saturday afternoons.

He says the difficulty with dealing with parents' emails is that it can be such an open-ended dialogue.

How do you go about answering emails about "who got the parts in the Christmas play"? It's about diplomacy as well as directness and emails run the risk of being interpreted differently from how they were intended.

Carl Ward, head of the City Learning Trust, in Stoke-on-Trent, says schools need to make sure that staff are not under any pressure to answer emails out of working hours.

Image caption Big companies and start-ups put their technology on show to 34,000 visitors at the Excel Centre

He says that it can also be a good idea not to give out individual staff emails and to filter requests through a central point of contact.

It's also not just a problem for teachers in the UK.

Lotta Juvin, a school principal from Malmo, in Sweden, says fears of a work overload is damaging the recruitment of teachers in her country.

Image caption The Pi-Top robotics devices were on display at the educational technology show

"Parents think they know everything," and they are not shy of sending in their suggestions and expecting replies, she says.

"Why didn't my child get all As?" she says, describing the questions in the email in-tray.

Schools are now expected to act as welfare centres and social workers, she says, adding to the type of out-of-hours requests likely to arrive.

As a school leader, she says, she has to lead by example by not sending out unnecessary emails. "You can say, 'No,'" she says.

Meanwhile, one other teacher at the educational technology show says there is also another category of irritating email sent to all staff: "I've lost my cup. Does anyone know where it is?"