Learn Spanish (Latin America): Queremos concentrarnos, así que dejen de ser maleducados. - We want to concentrate, so stop being rude.

The structure dejar de followed by an infinitive (here, ser) means "to stop doing something." In this sentence, dejen is the plural command form (ustedes), telling a group of people to stop.

Could I use parar instead of dejar de to mean "stop"?

When you want someone to stop doing a specific action, dejar de + infinitive is the correct choice. The verb parar is generally used for halting physical movement, like stopping a car.

Why is nos attached to the end of concentrar?

In Spanish, the verb for "to concentrate" requires a reflexive pronoun: concentrarse (literally "to concentrate oneself"). Because "we" want to concentrate, we use the pronoun nos and attach it directly to the infinitive.

Why do we use ser with maleducados? Aren't they just acting rude temporarily?

Even though they might only be acting rude in the moment, maleducado (literally "poorly raised" or ill-mannered) describes a person's character in Spanish. Because of this, it generally pairs with ser rather than the temporary estar.