If you have a legitimate reason for the non attendance of your child at a detention, like a pre-arranged dental appointment or something similar, the school should re-consider changing the date of the detention.
Unfortunately, the Dept of Ed's rules are none too precise. You are entitled to 24 hours notice and can object to the detention if it falls on an important religious day for your family.
You can also make changes if you're concerned about the length and safety of the route between school and home if your child were to serve a detention.
Thirdly, if you cannot be reasonably expected to make alternative arrangements for collecting your child from school, you can change the date of the detention.
Hasn't the school provided you with a booklet that has details regarding such matters, because they certainly should have done so?