My wife is an English teacher here in Thailand. She already has her 1 year non-immigrant B visa (first received 90 days, then extension to 1 year). I have 90 days non-0 dependent visa and now would like to extend it until the end of my wife visa. What documents should I bring to the immigration apart from the standard one such as both passport copies, my wife work permit copy, marriage certificate and the application form(tm.7)? Do I need any documents from her school?

You will need copies of her passport photo page and extension of stay stamp.

Some immigration offices will want a letter from her school stating she is still working for them.