"Los estudiantes firman la petición."
Translation:The students sign the petition.
'Firman' also means 'are signing' and/or 'do sign' (that's the 'English' of it, Duolingo)
You are correct, Alan. It can mean 'are signing' (moderators here have said as much, as well as my grammar books and teachers in the USA and Mexico), but Duo chooses to save the progressive present for the 'estan firmando' version, which emphasizes that they are signing NOW