This is a hard one. We're in a similar (ish) situation with our Yr 6 DD. Her last JA session is the same day as her local dance school summer show. We're planning to try and do something with her JA friends before then, but there was no choice about missing her dance show. It really will be a shame, but she has got MA's so will still see a few of them next term anyhow.
Also, one of her school friends parents has arranged a photography session for the girls in our local park when they finally break up in July, but the date was during WL summer school which DD is going to! So we had to say no to that - although I think they're trying to rearrange the date so she can be there! Our children do make big sacrifices for their dance, missing parties etc,but in the main, DD wouldn't have it any other way!
Perhaps, as others have said, you could get her close friends together and go for a big picnic or something, just before the end of term, so she doesn't feel like she's completely missing out.