I did that too but not with textbooks.  In my case, I often found the teachers were the worst offenders.  I would look at written homework assignments with misspelled words and confusing sentences.  I would correct in red ink and initial it.  This way, the teachers were made aware that I was paying close attention to what they were doing.  I never missed any parent/teacher conferences and in every case took my kids with me.  I wanted my kids and their teachers to see that I was involved and interested in what both were doing and made it a point to give my phone and email to each teacher.  I told them in front of my kids that if they were having problems learning or causing problems of any kind I wanted to be notified immediately.  It put everyone on notice that we took our kids education seriously.  I wanted the teachers to know me and my kids to see me interacting with them.   This took effort and time but was well worth it.