Learn Spanish (Latin America): Apenas empezó el simulacro, evacuamos el colegio. - As soon as the drill began, we evacuated the school.

When apenas (as soon as) refers to an action that has already happened, we use the indicative mood. Because the drill already began in the past, we use the preterite empezó. You would only use the subjunctive if the action were pending in the future, like if you were saying 'As soon as the drill begins...'.

Doesn't apenas usually mean 'barely' or 'hardly'?

It can! However, when used as a time connector introducing a clause, like in this sentence, apenas translates to 'as soon as'. You will often see it used this way to show that one action happened immediately after another.

Is evacuamos in the present or the past tense here?

It is in the past (preterite) tense. For the nosotros form, regular -ar verbs have the exact same spelling in both the present and the preterite. We know it means 'evacuated' and not 'evacuate' because the first action, empezó, sets the context in the past.

Does colegio mean 'college' like a university?

No, colegio is a false friend. It means 'school' (usually referring to primary, middle, or high school). If you want to talk about a college or university, you would use la universidad.