My boss congratulated me for the offer, said it's a pity that I leave AND spoke with HR so that I do have a shorter period of notice (otherwise I would have had problems). All on the same day. While accompanying me on my way out he said that especially with students, he invests in people. The doors are always open and he would be glad to see me there again. In fact, he recommended a former student worker to join another company before graduation in order to have a better looking CV. Im kinda blown away by this non-selfish (in a company's view) behaviour and strongly consider to rejoin at some point.