Schools can put them down as educated off site and there are specific codes that they use for the purpose so it doesn't affect their attendance records.
Having done a number of WE auditions with DD, the first rounds can be very quick - they see the kids in groups of 10-15, they run through the song once together, then each child sings a couple of lines, a couple in each group will be handed a recall letter and everyone else goes home. You are literally in the room for 15 minutes total.
If you get through to later rounds or finals, then you can be there the whole day.
DD was up for one the other week, so I emailed the HT and said she had first round late afternoon so would be missing the last 30 minutes of school, but if she got a recall then she would be out for some or all of the following day and possibly also a part or whole day the following week. In the end she got down to finals so was out for 1.5 days.
I would really recommend not fibbing to the school - it's hard for children not to talk and the amount of time off they will need off if cast means that the CD and council will want a letter from the school saying they are happy for them to take part.
Speak to the school and get their approval upfront. If they're not happy then you may need to make a call regarding the school.
DD's school are very supportive - partly because they think it is a 'good thing' and partly because DD is aiming for vocational school so it's as important to her as English and Maths. However we have an agreement that she only goes up for big feature films, TV and West End stuff in term-time - adverts, local theatres or student films have to be in the holidays.