Is it permissible to use descriptions such as "O short one" and "O thin one" when joking with friends, if they do not get angry, and is there any sin incurred by laughter mixed in among master's students when discussing knowledge, while observing Islamic guidelines?

There is no harm in joking if it is free from lying, scaring a Muslim, or harming him. So, if your friend does not mind you calling him "shorty" as a joke, then it is not forbidden. As for mixing in universities and the like, it is a cause of corruption, and whoever is compelled to do so must fear Allah and deal with women with seriousness and firmness, and not be expansive in conversation or joking with them.