Kelly Wilcox teaches English in a secondary school in south London. Her students are thriving. She has a boyfriend and a cat and a job she loves. She grew up in Connecticut in the US, but now her life is here. At least that’s what she thought.

Under immigration rules that come into force next month, skilled workers – including teachers – from non-EU countries will need to earn at least £35,000 to remain in the UK permanently.

With a salary of just over £29,000, Wilcox faces having to leave the country - and her beloved pupils - at a time when headteachers are facing a desperate shortage of teachers.

Many schools have had to look overseas for teachers to fill their gaps – to the US, Canada, Australia and beyond. Now they face losing them, with tight budgets making it impossible in most cases to stretch to £35,000 salaries – all in the midst of a domestic recruitment crisis the like of which has not been seen for years.

Shannon Harmon from the Stop35k.org campaign said: “The new rules will impact classrooms up and down the country. The average teacher’s salary in the UK after 10 years is £29,500 according to a 2013 OECD report, significantly short of the required £35,000 threshold. Who will replace these teachers?”

Wilcox, 26, studied English and creative writing at Westminster University before doing a PGCE to train as a teacher. She now teaches English to pupils from years 7 to 12 at Greenshaw high school in Sutton and she’s angry she stands to lose everything she’s worked for. “Teaching is a career that requires heart and soul. I am tired, I spend weekends and holidays marking or writing lessons, but the kids I do it for give me a reason for getting up to go to work at 6am every morning.

“The £35,000 rule undermines my efforts. It suggests I am unnecessary. It demeans me to a number.

“My salary does not reflect my worth or value. It does not reflect the impact I have on students. It only reflects how much I contribute to tax. I don’t see why I should be punished for my salary.”

Another American (who didn’t want to be named), a modern languages teacher at an academy in Berkshire, was equally furious. “I’ve devoted the last three years of my life to building a teaching career in the UK.

“I’m an experienced teacher of 38. If I have to leave the country at this point in my life I would probably have to go back to my parents. I’ve got no savings – I spend 72% of my income on rent and council tax. I really like my school. I would like to stay. It’s incredibly unfair.”

With a salary of just over £31,000 she is hoping a pay review later this year could bring her up to the £35,000 threshold if she takes on additional responsibilities.

“The issue for me now is all the uncertainty,” she says. “The government is making decisions without clearly thinking through the implications.” It will not only disrupt her life, she says, but that of her school – which like many others is experiencing recruitment difficulties – and her pupils. “I’m angry because of the injustice.”

Teachers of maths, physics and chemistry are exempt under the “shortage occupation list” because of the particularly acute shortages in these subjects, which have been acknowledged by the government.

Headteachers and union leaders argue, however, that there are problems recruiting in many other subjects and some say the salary threshold rule will make it virtually impossible to run schools that are heavily dependent on non-EU migrant teachers.

Vic Goddard, principal of Passmores academy in Essex, regularly recruits from Canada, sending his assistant principal every year to do a tour of Canadian cities to find teachers. He has five teachers from Canada, and others from Australia and South Africa and says the policy is “ridiculous”.

“Potentially I could lose 8%-10% of my teaching staff in a flash. I can’t believe they think they are going to be able to do that and us survive.

“I’m sort of not taking it very seriously at the moment. It would make my school almost impossible to run very quickly. It’s simply unworkable. Lots of schools will be affected. There are shortages in every subject. Maths and science are just a drop in the ocean.

“We’ve got so many other things to worry about – it’s just a ridiculous policy that was based on getting a headline. It’s not about common sense. It’s not about education. It’s a headline grabber.”

Kevin Courtney, deputy general secretary of the National Union of Teachers (NUT), is calling on the government urgently to reconsider the policy. “When Theresa May published her statement of intent in February 2012 the teacher recruitment crisis was not as acute as it is now. It seems absurdly counterproductive to force schools to dismiss teachers they’ve trained and invested in, and who are still very much needed, at a time when highly skilled qualified teachers are in great demand.”

The NUT also wants the government to look again at the shortage occupation list. “Currently, only secondary school maths, chemistry and physics teaching is considered a shortage occupation. However, headteachers up and down the country are struggling to fill a variety of teaching posts, not just these three subjects.”

Courtney says: “Migrant teachers make a significant contribution to the UK. Most of those affected by these proposals are Commonwealth or US citizens. These teachers and their families shouldn’t have their settled lives in the UK destroyed by a government which is now making them pay for the false promises of the past.”

A government spokesman defended the policy, saying that in the past it had been too easy for some sectors to bring in workers from overseas rather than to take the long-term decision to train the domestic workforce in the UK. “These reforms will ensure that employers – including those in the education sector – are able to attract the skilled migrants they need. But we also want them to get better at recruiting and training UK teachers first.”

Such words ring hollow for Wilcox and others like her. “It’s a strange feeling, to face being forced to move back to America,” she says. “I love where I grew up. But my life is not there. My friendships from America are purely Facebook likes and comments. My real relationships are here in London. I just absolutely love it here.

“I always knew I wanted to be a teacher – both my parents are teachers. I love teaching and the school has been so kind to me. I would be devastated if I had to leave.”