Is she doing compulsory 5/4 with a view to continuing on the elite path?
My thoughts would be that it's not really necessary to skip school until a bit higher up- level 3/2 at least. However I do realise many clubs like to take advantage of the quiet gym as they can be much more productive in that time.
It's also a year away. A lot can change in a year- you/she may not enjoy the high pressure of elite track. Some gyms will not allow you to continue on to level 4 if you're not going to do exceptionally well. I wouldn't worry about it until after level 5 vols this autumn- your gym may switch training times on you before then anyway!!
My dd is 9 and we will likely take her out an afternoon at some point. We had a quick chat with the head who is fully behind her, and will arrange class schedules/catch up sessions to suit, which is brilliant. Apparently it reflects well on the school and is part of the gifted and talented programme so it's brownie points for them.
Like I said, I'd just hold on for a bit, and maybe think about how you feel. Our pov is that elite sport is a huge opportunity, and one afternoon off primary school won't hurt her education long term. My mum, however, thinks we're mad, education comes first, the chances of making elite are too small, etc...
Is she at a uk elite hpc club?