Algunos alumnos se levantan muy pronttemprano, Aa las 5 de la mañana.

a = at. "a las 5" = "at 5" (in this context there's no English equivalent for the article "las", but it's required because "cinco" behaves as a countable noun when it refers to a certain time of the day). Also, not using "a" would sound as wrong as not using "at" in English. Imagine saying "Some students wake up very early, 5 a.m."