Well, choosing which tense to use depends on what you want to say!
There are no right or wrong tenses - the tenses let you express when something is happening and how long it went on for - or will go on for, like this:
Present tense: it is happening now (eg 'is')
Past tense (went on a while): it is already completed (eg 'has been')
Past tense (was over quickly): it is finished (eg 'was')
Future tense - hasn't happened yet: (eg: 'will be')
Here is an example of when you would use each one of your choices, which will hopefully let you choose which tense to use, according to what you want to express:
In response to the question, right now: 'hey! when's the festival?'
- The festival is scheduled to take place next Sunday
If you are writing a newspaper article, reporting what the festival organisers are up to, then use:
- The festival has been scheduled to take place next Sunday (note - it's a bit dry, reporting style)
If the festival has been called off, due to rain:
- The festival was scheduled to take place next Sunday (implies it's not happening any more, because of the 'was' - past tense. And the scheduling activity is also now 'over'.)
If you are not sure yet of the schedule, or the schedule has not been announced yet but you know it is going to be, then use:
- the festival will be scheduled to take place next Sunday
And with your second example:
In response to the question, right now: 'Oh, what's happening with today's lesson?'
- Today's lesson is rescheduled to next Monday
Announcing, over the school broadcasting system, to 'report' on what has been going on with the scheduling, (the scheduling itself, is already finished and is completed (in a rather dry way):
- Today's lesson has been rescheduled to next Monday (past tense)
In response to the question 'what happened to today's lesson?) (past tense)
- Today's lesson was rescheduled to next Monday
Where you are not sure of the schedule or think it will be rescheduled but it has not been announced yet, use (future tense, conditional):
- Today's lesson will be rescheduled to next Monday
I am not surprised that you are confused by the way - the descriptions of tenses that I have so far found seem .., awfully muddled and complicated. Maybe someone has a great definition of the various tenses that they can post as a comment.