A student Visa or F1 Visa is granted to students and is valid generally for the period of study (including grace period). Almost all these Visas will stipulate that you cannot take up a job in that country during the period of your stay.
The Visa granted to a Pakistani student planning to join a US university falls under the “Non-immigrant” category, and is known as an “F-1 Visa”. The important stipulation of this Visa is that its holder cannot take up a job outside the university campus during the period of study and that he/she should leave USA at the end of the period of study or if the study is discontinued earlier.
Local immigration authorities in USA can permit the student to take up a job outside the university campus during the second year of study provided the student can convince the local immigration authorities that his/her financial situation has changed dramatically necessitating his/her having to take up a job to finance the studies. (F-1 Visa holder can, however, take up jobs within the university even during the first year. It is possible to earn upto $500 per month from part-time jobs that one can get within the university).