This just shows you that you and your class are the same as every other one the world over.  It also shows that your test is probably fair too as if all passed, then it would be too easy.  Don �t look at the test as a reflection only of your teaching style but as a diagnostic tool to show you where you need to put in more work for your poorer students.  Although it is extra work, it would be good to split your class into two groups.  While the brighter ones are doing some kind of follow up work to your lesson, talk to the group who are struggling and try to give them some work based more at their level.  This will pay off long term.

Think of a builder.  He always starts with the foundations and works his way up.  If he started half way up the wall, then it would collapse.  If students are failing their tests then you must work more on the foundations rather than try to continue building the wall.  In reality there may be some students who never progress much further than the foundations.