I would not recommend Migros Klubschule. I am aware they are very cheap, but my wife made a course with them (German for beginners) and her experience was not good. The classes were really slow and mostly they were losing their time doing silly games. If I would be you (I also studied at ETH) I would take one of the courses at the University of Zurich and study a lot by my own (this is key when learning a language).
Also, I recommend you to do Tandem-Learning. The University of Zurich offers the possibility to register in a pool of people looking to do tandem and when they find a match they will contact you. Here is the link to register
. I did it as a student and helped me a lot! Good luck!