This depends on whether you are stressing the event or the reason.
The reason would be better given with the present perfect because the decision to cancel was made in the past, but it is relevant now.
e.g. When talking about the prime minister's visit you can add 'Oh, and because of this today's lessons have been cancelled.'
However if the sentence is used in the context of the event, then we would use the present.
e.g. 'Today's lessons are cancelled. We can all go to the beach.'
However while it is better style to use the statements and tenses in the context above, you will find many English users who don't do so. It is more a matter of style than grammar here.
(Though note that these are statements, and not 'questions' as you accidentally wrote.)