Learn Portuguese (Portugal): A nossa turma era muito grande, não é?

In English, the tag matches the verb's tense (wasn't it?, didn't you?). In Portuguese, não é? is a fixed expression used to ask for confirmation after a positive statement, no matter what tense the main sentence uses. You can think of it as meaning isn't that true? or isn't that right?.

Why use era instead of foi for was?

Era is the imperfect past tense of the verb ser (to be). We use the imperfect here because we are describing a characteristic or an ongoing state in the past (how big the group was over time), rather than a single, completed event.

Is turma the same as aula?

They both translate to class in English, but they mean different things. Turma refers to the specific group of students you study with. Aula refers to the actual lesson or the period of time you spend learning.

Why is there an A at the very beginning of the sentence?

In European Portuguese, it is standard to put a definite article (o, a, os, or as) before a possessive word. Because turma is a feminine noun, we use A before nossa to say our.