Student visas are possible to obtain upon filing a completed I-20 form with the US Embassy. Said form is obtained from the school and can only be issued for duration of studies and after providing school with name, date of birth, country of birth, country of citizenship, and latest INS requirement - proof of financial ability to cover not only the school costs, but transportation to and from home.
Even then, there is no guarantee. No matter how many "invitation letters" may be provided, if the applicant is unable to demonstrate a strong probability of returning home after completion of studies, the Visa will not be issued.
Link to INS site: http://www.ins.gov/graphics/index.htm
[This message has been edited by klcman (edited 09-13-2002).]