I learned it from my German teacher as muß. But I'm reading a lot of notes on the Internet that say it's muss and others that it's muß. Has there been an official change in the German language recently or something?
There has been a spelling reform some 15 years ago. Since then it's supposed to be
because the "u" is short and closed. Today, "ß" only follows long vowels (for example Straße or Maß) so "muß" would technically sound like English "moose".
However, a lot of people are still used to the old way plus the reform wasn't exactly whole heartedly received which lead to a good deal of confusion. So some youngster in a forum might actually write "muss" and then correct himself to "muß" simply because a vague awareness that there has been a confusing reform so the first impulse was probably wrong.