Learn Dutch: Wij wachtten buiten het lokaal. - We waited outside the classroom.

To form the simple past, we take the stem of the verb. The stem of wachten (to wait) is wacht. Because 't' is in 't kofschip, we add -te for singular or -ten for plural. Since the subject is wij (we), we add -ten to the stem wacht, which gives us the double 't' in wachtten.

Do I pronounce both t's in wachtten?

No, you just pronounce it with a single 't' sound, exactly like the present tense wachten. When spoken, Dutch speakers can only tell if you mean 'we wait' or 'we waited' from the context of the story. The double 't' is only there to make the past tense clear in writing.

Is lokaal the standard word for 'room'? I usually use kamer.

Kamer is the general word for a room in a house or hotel (like a living room or bedroom). Lokaal specifically refers to a classroom or a teaching space in a school or public building.