The thing is, I can't understand what you are trying to achieve by not telling the school.
Do you think that if your child isn't in school, but they don't know the reason, you will be given 'the benefit of the doubt' and your child marked a authorised absence? Er, no. The only possibility you have of that is if you ask, upfront. It is unlikely to be approved, but it might be.
Are you trying to avoid embarressment, or poossibly pointed questions, about taking unauthorised absence? Saracen's idea of a phone call or note dropped in just before you leave is a good one to avoid that.
In almost any course you take, the result is the same (an unauthorised absence noted in the register). However, the 'mood music' around that absence in the school could vary from really quite irritated (office staff spending 15 minutes each morning amking multiple phone calls, possibly escalating it if they are in any way worried about you) to entirely resigned, even happy (you request authorisation, with an apology around the mix up of dates, they know that e.g. the last few days are 'off timetable', and although they cannot authorise may be neutral about your absence). Unless it is your youngest's final term in the school - and even then, it would be common politeness to inform them - I'd just tell them. The Friday evening before, if you must, but still inform them so that no time is wasted chasing you up.