I have a US passport and long-term student visa for France. I want to go to the US for my spring break (half way through my visa validity). My visa is valid until July, I would go to the US for one week in April.
Will I have any problem?
Whether you can exit and re-enter France depends on more circumstances than you have specified.
Long-stay visa holders (including students) must, within 3 months of their arrival in France, submit their information to the Immigration and Integration Office (OFII). Upon submission of their information, they are then called to an appointment at OFII for a medical examination, and are given a 'vignette'. An example of such a vignette is here:
This is quite confusing, because technically, your visa will only allow you to enter the country for 3 months (even with a multi-entry, year-long student visa). After that point you require your vignette as well as the visa in order to enter. This remains the case until your student visa expires. 3 months before that happens, you should apply for your titre de sejour, which replaces both, and is a small card separate from your passport.
So, if you have your vignette and your visa in your passport, you can exit and re-enter France, there should be no problems. If you do not, things may become more complicated. I have heard that in the worst case, one would need to re-apply for a totally new visa.
However, if you are a US national, you have visa-free entry to France anyway. I do not know whether, at the border, the official would let you in or not without the vignette. I never tried to do that personally.
No you should not have any problems if you are still enrolled in school and abiding by the conditions attached with your students visa and as expected and you have confirmed, it is a multiple entry visa.