So I attended Tel Aviv University/ The Sackler Medical School American Program in Israel. I was born there (leaving at age 3), but returning every several years. I was initially concerned over having to do army time. After looking into the situation... I basically found that Israel would occassionally ask some students to participate in a 3 month training camp at some point in time... perhaps during the summer time, between a year of study. This option, however, was only taken by the unfortunate few who felt obsessed to simply 'get it over with.' You do not have to serve any time at all if you simply relax and do several important things.
1. Every time, before returning to Israel for the academic year, visit the Israeli consulate in the US order to get an army waiver. They should give you a waiver for at least 2 or 3 years. Probably not for every single year.
2. The time you may get questioned is when you leave Israel - not when you arrive. I was never questioned except for the end of third year. By then... I already had my passport stating that I had come and gone several times without any problems. I also simply pretended to not know any hebrew at all. I played it up as a total American. do not start showing off your potential hebrew speaking abilities. You should then have no problem at all.
3. If you start to freak before you go, you can ask the consulate to arrange an appt with the consulate general - or some visiting top level military person. But this may also backfire as simply a deal to delay duty - with duty nonetheless.
In the end only 2 students in my class served time at some point during med school because they tried to resolve the issue while being in israel - not a smart move. they served several months in training. Other students were told that they would have to serve some time after finishing medical school - upon some later possible return to Israel - see ya. Then - most - like myself, simply took advantage of the consulate letters - then played stupid at the airport - and - with some luck, made out fine - and loved every year of being in israel for medical school. Absolutely loved it. Enjoy...