Office Assistant ( BS-14 ) at Civil Secretariat, Peshawar will earn you more respect and connections in society . You will meet mature people as well as CSS/PMS officers and you will learn a lot from them which will give you confidence and motivation towards your real goal for CSS.
Another factor is that of place of posting in the job of Computer Instructor ( BS-17 ) at FG schools. You may be posted in city away from home. Even if not, you will always live under fear of transfer. While job of Office Assistant ( BS-14 ) at Civil Secretariat is non-transferable.The monetary benefits of BS-17 will be cancelled out with issues of living in another city and always hanging sword of transfer.