OK, only recently I was asked by an a trainee teacher to check her lesson plan. It wasn't big or complicated but as no one had done what she was hoping to do (due to the software being new) she wanted to check with me that she had the technical details correct. I sat with her showed her how to use the software and even wrote up a bullet point user guide. She took this on board and developed into her lesson plan. I was available if things went wrong or any problems that arrose, but in no way did I write a lesson plan.
I am employed to keep the network up and running to ensure that the teachers can use it to teach. How they choose to do this is entirely up to them. I however, do think that if I can supply training & user guides to our software and equipment that is what I should do.
I would take another look at your job description. If it is not in writing then you dont need to do it. If the teachers in your school are not capable of creating lesson plans incorporating ICT into their subject then it comes down to a staff training issue.