I don't understand the point of this thread. If you have a chance to go on a "once in a lifetime" trip, and considering not going because of 2 days of school, which is 8 hours of study (if that due to mucking around in class an such) it must not be a big trip. Sorry for sounding like a dick, but please go on your trip and study some other time. I went on an overseas performing trip with my school for a month in year 11 (I know it's different from year 12), but I could still make it up by doing some study before hand and during the trip. You can too. Don't miss out on this experience because of something that you can make up quite easily. My 2 cents.
i completely agree!
don't stress over it
get the work from ur teachers well before u leave. read through ur textbook and complete it in advance.
ask a friend to write down key points mentioned in that class or just ask the teacher to email it to u [believe it or not, teachers do actually plan the info they need to tell u!]
when you get back, if u are still really concerned, arrange a lunch time session to catch up with ur teacher on what u missed.
but seriously, don't waste time stressing urself out over nothing.
in the time u spend reading this thread u could have completely the work
good luck!