You are not alone with this. We have four kids, we have pulled them out of school for entire terms while we spent winter at Mount Hotham / Dinner Plain.
We have found this is a policy from the department, rather than the schools themselves. If you talk to your kids teachers, they will most likely agree with you 100%.
I believe this policy is in place for the lowest common denominator.
On our last winter on snow, we took the high school kids back to school for one day a week so they could catch up with their teachers and get more work etc etc.
All of the kids, passed all of their classes no problems.
We did strike problems when year 11 & 12 subjects became involved, all three started those in year 10. In Vic you have to be in attendance 90% of classes or FAIL in years 11 & 12. So that put a stop to long times away

Only two more years and we are over that for a few more years.
The little one is in grade four. We recently took her to Tassie for a week. Had a quick chat with her teacher before hand and included things on the itinerary that reflected what was happening in the curriculum at school for that time and coming up.
What do you know? She is well ahead of her class mates on those topics. Amazing what some 'real life' experience will do.
I say DO IT, you & your kids will be better off for. There is no way they could ever learn what that trip will teach them in a classroom.