I'm not exactly sure but from the 'cadetfaq.pdf' and 'cadet_handbook.pfd' files available on their website, it only said 'good mark' in both subjects.
I'm aiming to get the highest marks that I possibly can get until my final school exams next year.
I loosely know a REX pilot who went through the cadetship. He applied for it a few years after leaving school and got his rpl beforehand. During the interview (note that I don't know how they looked at his application before the interview), he said they showed more interest in his flying skills than his academic results.
For me, I'd assume that if you can get 85-90% on your exams or above, you should be in a pretty good position. I've also been told that it's a good idea to get some (~5-20 hours or enough to go solo once) in light aircraft beforehand as (if you go well,) REX are interested in seeing 'actual' performance in flying. (And while it may not reflect well in their simulator session, if you can read a chart and the weather and retain the knowledge from your flying lessons, that should help you.)
None of this discounts the fact that you should get the best mark possible in school but don't forget that it (probably) isn't everything.