Audtions for drama schools take place throughout the academic year. You do make the application in year 13 (if you are planning to audition while still at school) but there are things you could do in year 12 to lessen the stress/workload in year 13.
When auditions happen (for drama courses):
If you apply early (e.g. in October) then you are likely to get early first round audition slots. If you apply later (e.g. December) then you will get first round audition slots later. Each school has its own "rules" about applications and deadlines - and its own audition schedule. Some schools may offer places immediately, with a first and second round audition on the same day, others have several rounds of auditions (e.g. RADA has four rounds). I didn't apply until March - had my first round auditions in April and and second rounds in May. DS applied after Christmas - had first rounds in January, February and March and subsequent rounds in April, May and June (at different schools, obviously!

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If you and/or dd want to go to open days,I would recommend going to them as early as you can - it will probably help with the decisions about where to apply if she hasn't already made up her mind. It would also mean that you wouldn't have the added disruption of attending these *and* attending auditions in year 13. You might also want to consider seeing some of the final year shows at the schools if you can - and to make the most of that you would probably need to do that in Year 12. You don't have to go to open days - it won't affect the panel's decision about whether to offer a place - but it might give some useful information.
As a few people who have been through the process have mentioned - probably best to start preparing the spreadsheet and folders early! You do need to look at each school's requirements individually.
Your dd does not have to audition in year 13 - she could wait to apply until she has left school. My ds did audition in his A level year and seemed to cope OK with the workload - though I couldn't hand-on-heart say that he worked flat out on his academic work...

He didn't go to any open days and he didn't see any final year shows - but he had clear ideas about which schools he was interested in so I left him to it!
She could definitely spend some time in year 12 (and the summer holiday) on the look-out for great speeches to use in auditions. Doing research in to the requirements for various schools and starting the search early could really relieve the stress of looking for them and learning them in the autumn of year 13.
I think the useful preparation for year 12 is a) researching the schools and their requirements and b) reading plays, going to the theatre, looking for speeches, working on fitness/stamina, doing impro. All of that can be fun though - so it hopefully won't add to stress levels
