Students at any level are not allowed to work full-time, while studying in the USA. However, employment of international students, who reached USA on a non-immigrant visa such as F-1, is strictly regulated.
If such students maintain their visa status and be in good standing, they could work part-time and on campus only for upto 20 hours a week. During holidays and college breaks, students could work full-time, if they maintain the good standing.
Under special circumstances, students could seek approval from the Department of Homeland Security (through their respective college) to work full-time in case of 'economic necessity' or if they experience unexpected financial crises. Such full-time work is possible only after one year of stay in the US and after proper endorsement on the I-20 by the International Student Adviser of the university where the student is enrolled.
Even Off-campus employment must be authorized in writing.
After completion of education, any student could take up Curricular Practical Training (CPT) or internship, for a period of one year with companies in the USA, which is normally the first step to seek a more regular employment under H1B category, for further stay.