http://www.easywdw.com/category/calendar/02-february-2014-crowd-calendar/
calmom. this is a link to Josh's site WDW easy. he has crowd calendars. it's kind of a complicated chart, but once you understand it, it's a great tool.. this is of feb 2014. I couldn't find 2015.
it shows the crwod level see how the number gos up on president's weekend? and which parks are recommened for each day and which are not advised/
as far as weekends being busier, that isn't true. not like at a local park like 6 flags,e tc.
for example, Mondays are not the best days for MK, (it's usually people's first day there and they ususally go to MK first. and tuesdays are not good for epcot. peopel usually hit epcot second, and it's usually and EMH day. sundays are good cause it's a common check in, check out day.
I don't think saturdays are bad, as long as it's not a holiday weekend.
you could also pick up a copy of The Unofficail Guide to Disneyworld. it has a chapter onthis stuff.
I agree with yopu about missing school. even up to 3rd grade I would take them out. 4th adn 5th,if they are good students. middle school gets tougher and high school, forget it. (I know many will disagree and think it's bad to take kids out. I think it is well worth it JMHO)