I'm invited by my girlfriend in Germany and she is supporting me with everything. So I want to know if i will be granted the Schengen visa, if she is the one who write the affidavit for financial support?
Having €700 a month will not qualify her for a affidavit (Verpflichtungserklärung), unless she has a couple of thousand on her desposit. It is required to have an income of at least ~1300€ (depends on local authorities. It's 1479€ in Konstanz and 1290€ in Lippstadt). But 700€ monthly will not be enough.
However she can make an invitation without financial obligations. In this case it's you who must prove having enough money for the whole period of stay.
Depends on her financial circumstances. Being a student does not disqualify her, but a typical student could not afford to pay for your support.
Follow-up: The Verpflichtungserklärung covers not just the planned cost of your stay but also the unplanned costs for German government agencies if you can't or won't return home on schedule. So the planned length of stay does not matter. €700 a month does not leave much disposable income.