This article is from the X-Men Comic Books FAQ, by Kate the Short (firstname.lastname@example.org) with numerous contributions by others.
Apparently, Xavier's School for Gifted Youngsters offered an accredited
high school equivalency degree for its privately enrolled students. The
"graduates" could try for a higher degree, now that Xavier also has an
Institute for Higher Learning, if it wasn't for the fact that they're
really the X-Men, and usually have other things to do than cram for
finals. Some X-Men have tried for higher degrees--Jean Grey went on to
Metro University--but generally there wasn't much higher learning going
on at Xavier's. Some of the older X-Men have served as "guest lecturers"
to Generation X at the Massachusetts Academy, but it's pretty likely
that none of them has a teaching degree.
Scenes from Claremont's run on the New Mutants showed classes being held
in world history, economics, and physical education (above and beyond
Danger Room training). Presumably the classes in biology would be
top-notch. Even when she was with the X-Men, Jubilee was shown studying
Tom Galloway has more help: "It's strongly implied that Hank McCoy (the
Beast) earned his doctorate at Xavier's. He leaves the school directly
for what amounts to a job as a Principal Investigator at Brand
Industries, and there's really no time in his history when he could
otherwise have earned the PhD they credit him with." On the other hand,
X-Factor Annual #3 features a backup story where Beast is showing the
kids pictures of his time at a college outside of Xavier's, earning
degrees in biochemistry and genetics.
Don't mess with Marvel time, kids. These men are professionals.
As it stands, the promotional comic for Generation X had a whole list of
classes offered at Emma Frost's school. It included lots of specialized
PE, classes on leadership, cultural diversity, and physics with assorted
guest instructors. Still, Gen X first showed the kids in a classroom in
issue #21. More recently, both New X-Men and New Mutants showed students
in flight class, and Professor Xavier was lecturing on mutant rights and
politics, so students in Xavier's current school should be receiving
general and specialized curricula.