Learn Spanish (Latin America): El chico dice que lo de que nos multan por el casillero es real. - The guy says that the thing about them fining us for the locker is real.

Let's break it down: lo refers to the abstract situation ('the thing'), de links it to a topic ('about'), and que is needed to introduce a clause with a conjugated verb (nos multan). English often uses a gerund here ('about them fining us'), but Spanish requires que plus a conjugated verb to express this complex situational reference.

Why is nos multan in the indicative rather than the subjunctive?

Because the situation is being reported as a piece of factual information. The structure lo de que takes the subjunctive if it is tied to an emotional reaction or doubt (e.g., Me molesta lo de que nos multen - 'It bothers me that they fine us'). But here, the guy is simply declaring that this situation is a reality (es real), so the indicative is required.

Is multan referring to a specific group of people here?

Not necessarily. Using the third-person plural (multan) without a specific subject is a common way to express an impersonal 'they' in Spanish, just like in English. It refers generally to whatever authority is in charge of the lockers without having to name them directly.

Could we use para instead of por in por el casillero?