I know many teachers think this because it gets expressed to me numerous times during the school year by many different teachers. What some teachers fail to understand is there is a difference between 'social' language and 'academic' language (the language of text books and tests!) This is of particular importance to Stage 2 and Stage 3 teachers. I found this Youtube clip that might help to explain it more clearly.
If you would like more information please check out one of my original blog posts where I linked to a clip where Dr Jim Cummings, inventor of the terms BICS and CALP, talks about these terms and their implications for EAL/D students.