Breakdown of Они идут в класс четыре.
они
they
класс
class, classroom
в
in / to (direction)
идти
to go (on foot) / to proceed, go (of a process)
Questions & Answers about Они идут в класс четыре.
Why is it класс четыре and not четыре класса?
Word order changes the meaning! Четыре класса means "four classrooms" (talking about a quantity, which forces the noun to change its ending). But класс четыре means "classroom number four." When a number is used as a name or label after a noun, both words stay in their basic forms.
Why use идут for "are going"?
Идут is the "they" form of идти, which means "to go on foot" in one specific direction. We use it here because they are currently walking on their way to the classroom.
Does в mean "to" or "in" here?
Here it means "to" (or "into"). When you use в with a verb of motion towards a destination (like идут), it indicates direction and requires the following noun to be in the Accusative case.
If в requires the Accusative case for destinations, why hasn't the word класс changed?
Класс actually is in the Accusative case here! For masculine inanimate nouns (objects and places), the Accusative case looks exactly the same as the basic dictionary form. So класс doesn't change its spelling.