Imperfect or perfect

Actually you cannot choose. The first example needs to be imparfait (passive voice) and the second needs to be in passé composé. In the first sentence the phrase "tous les ans" points to an ongoing custom which asks for the imperfect tense in French. The second sentence (I assume, because it isn't quoted in its entirety) speaks about a single event.

In English you could translate the first sentence as: "The students used to be welcomed by the director every year." The phrase "used to be" as an attempt to capture the flavor of the imperfect tense.

There are some cases where you can technically "chose" between imperfect or passé composé. While you have to use the imperfect in "Les étudiants étaient accueillis par le directeur tours les ans" because it is a repeated action, as Chris already explained, you DO have the choice when it comes to the second sentence. Both "Elles ont été surprises par..." and "Elles étaient surprises par..." are correct. Etre is one of those verbs that is usually in the imperfect, but can at times be used in the passé composé.  Nobody would bat an eye if you used one over the other here, but there are nuances. Using the imperfect implies the duration while the passé composé makes it sound like it was a very brief, short lived surprise rather than an ongoing state. Now if the surprising happened repeatedly, such as every year at their birthday or something like that, you would not have a choice anymore, it has to be in the imperfect. 

I have just been marked wrong for Les étudiants etaient accueillis le directeur ce matin. .in favour of ont ete. What is going on?