Just to clarify what blondie wrote, (..."if you've done the prorrogas, and then after 6 months leave for 24 hrs
you can do the prorrogas again, ie. only have to go to Uruguay every 6
months"...) It is true that you only have to go to Uruguay once every six months, but one must do so 90 days after the prorrogas. So, yes, it is possible to go to the office of migraciones on Antardida a few days before the expiration of the 90 day tourist visa and get a 90 day extension at the desk of prorrrogas de permanencia. Then, after the 90 day extension, it is necessary to make the trip to Uruguay in order to receive a new 90 day visa upon returning to Argentina, thus starting the process over again. In other words, yes, you can spend a total of six consecutive months in the country before it is necessary to leave for Uruguay, but I don't think it's necessary to stay overnight.
I don't know about working with a student visa, unless you are required to get a DNI.
If you have a DNI (even with a visa rentista or pensiona) I believe that you CAN legally work in Argentina. You just can't vote or hold elected office.