I'm afraid that a lot of adverbs are on their way out. I'll wager that 90% of schoolkids in the U.S. wouldn't think anything was wrong if they heard, "I did good on my report card", or, "I didn't do too bad on my report card".
I also think that 'of' may replace 'have', when used with could, would, and should.
But you asked what has changed. Well, over-all, we've become a lot less formal, and in other things than language as well. How many letters today start with Dear Sir or Madam, or are signed, Very Truly Yours?
One thing your kids may be able to relate to is the "shorthand" that has developed with the use of pagers, or Leetspeak, I believe a previous thread called it. And don't forget words and phrases that have entered our language from movies; I'd be interested to know how many of your students recognize Here's lookin' at you, kid; whereas you and I couldn't have recognized Make my day.
