A book published not later than 1995 and probably after 1970 from age, type of cover, etc. The book could be young adult fiction or regular fantasy.
I remember no title or author or character or place names.
One of the first scenes is two friends practicing fencing at some kind of fencing school, probably in a college. One of them refuses a one-on-one challenge from the other that doesn't require a particular form to be used, commenting that the free style the other guy uses is too much for him. They repair to the house of one of them, where his family also lives, and night falls. Some kind of portal or portals open and dire-wolf-type beasts come through and attack the house. Everybody grabs swords and fights them off, and it is revealed that the family has had such contact in the past. At some point shortly after this the two friends either go or are taken through the portal in to some kind of fantasy world.
At that point I stopped reading.