Correct tense for "were welcomed"

Question

"The students were welcomed by the headteacher this morning."  Why is this translated by the passé composé rather than the imperfect tense?

Accepted Answer

If I may just add, Steve, the passé composé indicates an action in the past which is finished. The imparfait gives a sense of duration to the action and is often translated by 'was/were , being/doing ...'
In this case,
The headmaster welcomed the students in the morning ( active voice):
Le directeur a accueilli les étudiants ce matin.
or as in the text, 
The students were welcomed by the headmaster in the morning ( passive voice):
Les étudiants ont été accueillis par le directeur ce matin.
This is the same action viewed from different perspectives but in both cases the passé composé is used and we are assuming that the action has taken place earlier in the day.
Hope this helps!

Answer

Bonjour Steve,
Here is a very similar phrase using l'imparfait:
Les étudiants étaient accueillis par le directeur tous les ans. ---> The students were welcomed by the headteacher every year.
In this phrase, l'imparfait is used in the passive voice because it is a habit by the headteacher, he/she does this yearly.
Here is the lesson that explains this:
https://progress.lawlessfrench.com/my-languages/french/view/441?rts=%252Fsearch%253Fs%253Dl%252527imparfait
On the other hand, the phrase in question:
Les étudiants  ont été accueillis par le directeur ce matin. --> The students were welcomed by the headteacher this morning. is a once and done phrase, hence the use of le passé composé in the passive voice.
J'espère que ma réponse vous aiderait.
Bonne chance et bonne continuation dans vos études en français, la langue de Molière et qui a été utilisé par le monde français depuis l’époque d’Hugues Capet

Ron (also a non-native speaker)

Answer

Just to expound a bit on Ron's explanation:

Take the sentence: "As he was welcoming the students, his phone rang." Here the welcoming part is an ongoing process during which the phone rings. Therefore you would use imparfait for the welcoming part and passé composé for the ringing:

Alors qu'il accueillait les étudiants, la téléphone a sonné.