On has value of third person singular in French conjugations, but here barrés is the past participle of the sentence and is always in agreement with the subject, which is “On” in this case, which is plural.

So « Quand on s’est barrés de cette ville … » is right.

You have to differenciate the 2 possible usages of “on”:

Indefinite

On est souvent étonné du prix des choses.

You don’t know who “on” stands for. In this form you’re supposed to do as it was a masculine singular.

Replacing a personal pronoun (usually “nous”)

Je suis parti en vacances avec ma femme mais maintenant on est revenus.

In this case, “on” is used as a replacement for “nous”, the sentence could be written :

Je suis parti en vacances avec ma femme mais maintenant nous sommes revenus.

This last form is more formal though, and tends to be used more when writing, while the first one if more appropriate, more informal, less pedantic and then rather used in spoken language.

Note that the past participle is in this case accorded in gender and number with the subject it replaces. If “on” stands for 2 women, you should write :

On est revenues

It is worth mentioning that in this case, when acting as the subject to accord the auxiliary “on” is considered masculine singular, and acting as the object to accord the past participle, it’s important to know who it stands for, how many people and of which gender. As a result, “on sommes revenu(e)(es)” is NOT correct.