Como dizer "Aula vaga" em inglês

Na faculdade, diziamos simplesmente "class cancelled". Na escola, sempre havia um professor substituto ("substitute teacher") que preenchia o tempo, geralmente sem ensinar nada. Se fosse quieto, podia até jogar baralho.

Eu joguei um pouco de baralho numa classe de "probability and statistics", mas nunca fiquei bom nem no baralho nem na matemática.

I think it's "free period". Check it out: en.wiktionary

I think they would describe it as a "uncheduled time" (for shortage of teacher in the beggining of the year, in Brazil, I mean).
The teacher of a given subject is not yet available/hired, and each teacher is already teaching four subjects (teaching Maths and Portuguese, for instance).

In a given day, say the teacher won't come because something came up (his or her house flooded, etc) then it could be the case for a "spare" or "free period" or even "an off period".

Let's say you want specify it "We'lI have the first (second, third, etc.) period free. That is, a segunda aula, terceira, etc, será vaga.

Cancelled is to the case of all classes for the day being cancelled, kinda "day off". So you will have classes cancelled for snowstorm,

"Oct. 10 afternoon classes cancelled for Mean Green Football."

Due to snow, Saturday Classes are cancelled today.
Debido a la nieve, las clases de hoy, sábado,
17 de febrero de 2024 estan canceladas.
Source:Ridge Street Elementary School

Notice: In the US they hire assistant teachers, so in case a teacher can't 'show up' the other will cover his/her class, but 'not in a permanent situation, of course.

Henry, fique frio!

Pior foi eu, nem fiquei bom em baralho, nem estátística, e muito menos na 'cadeira' de horas vagas! E olha que repeti essa de Horas Vagas várias vezes.