A day earlier, in the hope of earning a pay rise that will give her stretched household budget some relief, the single mother of two will join more than 120,000 other teachers on the picket line.
Teachers across England and Wales are going on strike on Wednesday after a decade of meagre earnings in a state-funded school system that has seen many take up second jobs or leave the profession altogether.
"It's utterly heartbreaking for me," 38-year-old Preston said of missing her son's birthday. She works as an English teacher three days a week, looking after her kids on the other two days as she cannot afford childcare every day.
"If I could just make enough money to not have to do the tutoring in the evening, I would have a much, much happier life ... It's just a really, really depressing place to be in."
Hundreds of thousands of other workers including rail staff and civil servants will also walk out on Wednesday, making it Britain's biggest day of strikes in several decades when measured by the range of industries it will cover.
The National Education Union (NEU), which is organising the teachers' strikes, has asked for an above-inflation pay award funded fully by the government, so that schools can also cover other costs, from stationery to textbooks.
With inflation reaching double digits last year, teachers have seen a 23% real-terms pay cut since 2010, the union says.
Preston says mortgage payments eat into two-thirds of her 1,800 pounds-a-month ($2,230) salary, forcing her to find other ways to make money, such as renting out a room in her house to a lodger and buying cheaper, frozen food instead of fresh produce.
"The stress that causes is absolutely unbelievable ... Every single month, it is a struggle," said Preston, who has worked as a teacher since 2011.
The government, which has held unsuccessful talks with the NEU, has called its one-year, 5% pay award for teachers the highest "in a generation" and says it is investing 4 billion pounds in schools over the next two years.
The NEU - which has planned seven days of strikes in total - says one in four teachers leave the profession within three years of qualification, impacting the education of children.
"I can't remember when we had enough staff to comfortably cover the school," said Sydney Heighington, 33, an assistant head teacher at an east London school.
"At the moment, you've got teachers just absolutely leaving in droves," he added, noting that some of his support staff colleagues had been forced to go to food banks because of rising bills and others had simply left to find work at supermarkets.
Heighington, who teaches music, said more than a third of experienced full-time teachers and teaching staff had left his school last year. Only a fifth of those roles were filled — by trainee teachers.
Educators say schools having to pay teachers' salaries from their own pocket has left classrooms starved of money for textbooks, IT upgrades and school trips.
"You cut back on trips out, you don't go to the British Museum, you don't go and see stuff," said Steve Chalke, founder of the Oasis charity that runs more than 50 schools across Britain. "So subjects become a little bit more sterile, because you're not learning at every level."
Prime Minister Rishi Sunak wants to expand mathematics education in schools but the NEU says his plan fails to address teacher shortages, which mean one in eight maths lessons are taught by a teacher unqualified in the subject.
Reports say teachers at the elite Winchester College in southern England, where Sunak attended school and was a head boy, are among those striking on Wednesday. The school declined to comment.
($1 = 0.8076 pounds)