"Billy, you are dealing with the oddity of time travel with the greatest of ease."

This doesn't make sense to me:

Time travel movies like Back To The Future and Bill & Ted's Excellent Adventure wherein something has to be changed to keep things how they should be.

In B & T's Rufus came to help Bill and Ted with their history report because if they don't pass the class Ted will get sent to Oates Military Academy and Wyld Stallyns will never be, which is the foundation of Rufus' society (their music ends war and poverty, brings about world peace, aligns the planets, blah blah blah.)

Ok, so if Rufus is from the future and Wyld Stallyns exists then didn't they pass the class? This makes no sense.

Also, in Back To The Future II Doc comes to Marty (and Jennifer) so they can fix something Marty's son does. Yet Doc is always going on about not changing the future.

Don't get me wrong, I love both movies. But am I missing something? (Gosh, why does everything have to be so logical with me today?)

Ted: "I'm Darth Ted."
Bill: "Well I'm Luke Bill...and you're not my father!"
Ted: "Beat you, I will. Yes."
Ted: "I totally conquered you, dude!"
Bill: "I'll never rule the universe with you!"