I can answer your questions. Public schools in the LA area get off the week before Easter, private schools get off the week after Easter (and some private schools get the week before Easter off as well). From what I understand (I never go to SFMM during spring break), there are definitely crowds, however the crowds are not horrid. The crowds should be like a summer weekday with most of the major roller coasters having a 45 min -1 hour wait maximum (average waits around 30 min). Deja Vu and X will, of course, have longer lines, but the only way to avoid that is to ride those coasters first.
To answer your second question, SFMM is very nice to ACE when in comes to certain things. However, being an ACE member doesn't give you special privileges or discounts on regular operating days.
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http://home.earthlink.net/~sfmm84/ Six Flags Magic Mountain Xtreme Guide