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.
Heighington, who teaches music, said more than a third of experienced full-time teachers and teaching staff had left his school last year.
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.
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.