A kesher needs to work both ways – if a teacher doesn’t want one, you can’t build one! When I was in yeshiva, our Rabbeim used to do things outside the classroom with us also. In the spring, when Shabbos started later, one Rebbe would sometimes play basketball with us for an hour. Another Rebbe would sometimes host something in his home, such as a Chanukah mesibah, a Friday night oneg, or something else.
When I was leaving the yeshiva, I asked my Rebbe if he minded me calling him to keep in touch. He was happy to say yes. Now, almost 13 years later, I still speak to him, and when I’m in the area, I stop by to visit.