Multi-level classes

One of the negatives of teaching at my university is that students are grouped according to what major they take and not what their level of English is.  This results in classes having one or two students who are semi-fluent (having studied overseas perhaps, or at hagwons for years) mixed in with a few students who struggle to say their name and how old they are.  And the instructors are supposed to make one book fit all.

How to deal with this as a teacher?  It's not easy.  I struggle with it, even in my 4th year of having to work with it.  Essentially, I teach to the middle 80% of the class.  I know that the top 10% of the class will be bored with what I"m teaching.  If the student has studied overseas and is way above the class level, I'll often excuse them from actually attending and just make them do the homework and tests.  And I know that the bottom 10% of the class will not really be able to follow what I'm doing or participate in a meaningful way.  I usually leave these students to do their own thing as long as they don't disrupt the class.

And this also makes testing a challenge.  For the recent midterm exam, I did a speaking test, where I gave the students some sample questions that I would be asking.  I asked some questions straight off the study sheet word for word but changed some questions slightly (for example: What's your plan for after graduation? ---> What's your plan for tonight?  What's your plan for after English class?  What's your plan for winter vacation?)

For the top students, the test is almost edging into the ridiculous.  It really is way too easy.  And for the lower-level students?  Instead of asking some questions that have been changed slightly, I would ask the ones that came straight from the book, or study paper.  That way, if they really did study they would for sure be able to give at least some answer. 

What are your strategies?