I am a U.S citizen and have an expired Israeli student visa which I used to study in Israel for two years. I now plan to go back for another year of study. Will I be able to enter the country on my expired student visa?
No. The visa is valid for a fixed period of time, which I'm sure you realise as you use the term 'expired'.
It usually has a date on the visa, and if that date is in the past, you can not use it in the present or future.
You'll need to get a new student visa, or you will be turned away when you try to board your flight.