Your son has been assigned a detention to be served on a convenient day next week.
Your son was assigned a detention to be served on a convenient day next week?
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Grammatically speaking, either sentence works. However, to a native speaker, use of the present perfect (has been assigned) might sound more natural.
The past simple is used to talk about complete events at specific times:
I played football yesterday. I ate ice cream last Thursday etc.
The present perfect we use to talk about things that happened in the past, but that have an effect on the present:
I have been to the USA. I have been sky-diving.
In this case, since the detention has been assigned, but the actual detention is yet to take place, it may seem sensible to go for this second option and use 'has been assigned'.
For more information between the two tenses, see:
Both sound wrong to a native American English speaker. I would restate this sentence as any of what follows:
"A detention" is grammatically incorrect. "A detention day" or "a day in detention," however, would be correct.