Well, I wouldn't say that it's a passive form. Note that 'acudir' is not a transitive verb. You only can transform an active sentence into passive if the verb is transitive. In high school we study "se" forms" in depth (as Jpablo said, although I think we spent more than three days

).
There are two kind of passive sentence in Spanish, one of them is called
pasiva perifrástica, with the verb to be. This one doesn't have a very common use. Instead, we use very often
pasiva refleja, with "se". In passive sentences, the subject becomes the object (and vice versa):
Esta tienda necesita un empleado. Activa
Un empleado es necesitado por esta tienda. Pasiva perifrástica.
Se necesita un empleado. Pasiva refleja.
We use p. refleja because usually the object (subject in the active sentence) is not needed to know or we know it.
Impersonal sentences use se and their construction is almost the same:
Debe acudirse /se debe acudir al diccionario.
Se come muy bien aquí.
But there is an important difference which follows the rule "the verb must agree with the subject in number and person" (person will be third singular always, both in pasivas and impersonales. This will give you the clue to know if it is an impersonal sentence or a pasiva refleja one:
Se necesita un empleado - Se necesitan varios empleados.
Se acudirá al diccionario - - -
This
link will give you additional information and examples.