She is philosophical. She loves her job so she tries to prioritise the work and manage the stress. She works six 12-hour days a week, she says, and year by year the hours are getting longer.
Mrs Mills, 56, is one of two deputy heads at Whitchurch, Cardiff, which has 2,400 students and 200 staff. She is responsible for curriculum, community and staffing.
Her background is in teaching French and German but currently she spends five hours a week in the classroom teaching personal and social health education.
Today she is out on lunchtime duty to help cover a number of staff absences. "It's refreshing to get out there and see the students," she says.
And well it might be. Much of her day is consumed by meetings, typically 10 or 12, and paperwork in the office. She says: "Occasionally I think it would be nice to go home in the evening, to go to the cinema for once."
Mrs Mills is one of the deputy heads who are working dramatically longer hours, according to new statistics from the School Teachers' Review Body.
Since 2000, secondary school deputy heads' average workloads have increased from 58.6 hours a week to 61. Secondary heads have increased to more than 65 hours.
"There's been a spotlight on removing admin tasks from classroom teachers, which is good," says Mrs Mills. "But that means there are a lot more admin staff and teaching assistants who are a valuable resource but need to be managed."
Secondary classroom teachers' hours have decreased in the past six years from an average 51.3 hours a week to 49.1 hours.
Primary teacher Anna Goh noticed a difference almost overnight when, a year ago, she was suddenly able to delegate administrative tasks and spend two hours and 20 minutes a week on planning, preparation and assessment.
Mrs Goh, who is in her 10th year of teaching, is the literacy and assesment co-ordinator and head of Year 6 at Canon Maggs CofE junior in Bedworth, Warwickshire.
She says: "It means you've got that extra bit of time so you don't have to take it all home. You can give yourself a bit of time off, you can prioritise. I'm going home fractionally earlier and doing more at school. I can spend my time on the things I'm trained to do. You can make your teaching more effective for the children."
Mrs Goh says her headteacher, Rod Steward, is meticulous in ensuring staff took the time owed to them, and that 20 minutes a week of PPA time is banked so teachers could occasionally take out a whole day.
The review body figures show that primary teachers' average hours per week have dropped from 52.5 in 2004 to 50.9 last year and 50.1 in 2006, marking the introduction of workplace reforms.