Shilling

The shilling was a form of currency used in Great Britain up through the mid 20th Century. Its value is 20 shillings to a pound, and 12 pennies to a shilling. Susan Foreman, being a time traveller, had visited Britain's future before, and, simply being confused, could not remember how many shillings were in a pound when she and the Doctor settled down in 1963 London.

It would only be after 1971 that Great Britain would switch to the decimalized Pound, with the shilling now at 5 new pence. Susan probably didn't remember which year it was that this occurred, resulting in her ridicule when she thought that they had already decimalized the currency while attending high school.

Interestingly, with An Unearthly Child set and broadcast in 1963, Susan accurately predicts that England would switch to decimalization. Given the reaction of the teacher and students, it is most likely that the switch had not yet been announced yet.