The first thing to know is that students with student card cannot work full time or self-employed, as the school timetable may not match the working hours, so you have to choose to work part-time or full time but that does not exceed three months and after school hours. Similarly, the duration of the contract may never exceed the validity of your student card
The two options for work are:
The internship agreement: This is not an employment contract is an agreement on involving the university, the company and the student concerned itself. Most universities and business schools in Barcelona offer their students through practices that have signed agreements with companies that develop an activity in the sector are pursuing studies. In order to sign this agreement must be approved 50% of the credits you're studying and will offer financial reward as a study aid that in no case is a wage compensation.
The employment contract as a student part-time or fixed-term: to reach this agreement have to have your ID number abroad, your NIE and a work permit , you'll get only if the work is compatible with your school hours. Likewise you must take into account that revenues will never be the support to be able to live in Barcelona during your study period, but an extra and must ensure that you have enough money to study in Barcelona without having to need to work for that. Your work should serve to get work experience.
If you want more information on the necessary documents and request permis work here we leave you two links to access:
http://extranjeros.mtin.es/es/InformacionInteres/InformacionProcedimientos/Ciudadanosnocomunitarios/estudiantes.html