I spoke of a teacher who gives a three hour grammar diagnostic test. I tried a different route. I gave an essay writing diagnostic and I recorded each grammar and writing errors or proficiencies on a chart as I read each student’s paper. When I finished, I looked for patterns. Their diagnostic assessment took 45 minutes for them to do and about 30 minutes for me to read and record. I found that out of all of my students that there was only one “grammar”learning gap that three students shared and that was spelling. There were a few learning gaps that two students shared. The writing diagnostic revealed that they could use grammar fairly well; it did not interfere withe the comprehension of what they wrote. Their biggest learning gap was not in grammar but in their actual writing. About 80% of the class lacked specific examples to prove their points. I will focus on writing and teach grammar when I see specific needs.
What powerful diagnostics do you give?
Education with Technology Harry G. Tuttle 
Diagnostic assessments are essential instructional tools for effective English-language Arts and reading teachers. However, many teachers resist using these tools because they can be time-consuming to administer, grade, record, and analyze. Some teachers avoid diagnostic assessments because these teachers exclusively focus on grade-level standards-based instruction or believe that remediation is (or was) the job of some other teacher. To be honest, some teachers resist diagnostic assessments because the data might induce them to differentiate instruction—a daunting task for any teacher. And some teachers resist diagnostic assessments because they fear that the data will be used by administrators to hold them accountable for individual student progress. Check out ten criteria for effective diagnostic ELA/reading assessments at http://penningtonpublishing.com/blog/reading/ten-criteria-for-effective-elareading-diagnostic-assessments/ and download free whole-class comprehensive consonant and vowel phonics assessments, three sight word assessments, a spelling-pattern assessment, a multi-level fluency assessment, six phonemic awareness assessments, a grammar assessment, and a mechanics assessment from the right column of this informative article.
how far can teachers use diagnostic assessment in class six.
Duah,
I do not know what “class six” is. I use formative assessments in my college classes. I believe that educators at all levels should use formative assessments. I like to think of formative assessment as the apprentice system that used to be in the trades. Your mentor constantly assessed your skills to advance you to the next level.
Harry