Ron Dutcher wrote:

One of Mark Twain's school teachers was a Mr. Cross. AB Paine always
refered to him as "Mr. Cross." in his various Twain biographies.
Noted Twain researcher Ron Powers, in his book "Dangerous Waters"
names this teacher as "Frank O. Cross";however, in Power's most
recent book, "Mark Twain, A Life", he identifies this teacher as "Sam
Cross."

Errr....did Powers get his Crosses crossed?
If any one out there actually knows the correct name, or anything
else about this teacher, I would sincerely appreciate hearing from you.
In the biographical directory published by Dahlia Armon in Huck Finn
and Tom Sawyer among the Indians and Other Unifinished Stories,
Clemens's teacher is identified as Samuel Cross:
Cross, Samuel (1812-86), was seven years old when his family
immigrated to Pennsylvania from Ireland. He moved to Missouri in 1837
and by 1840 was a teacher in Hannibal. With John Marshall Clemens
[and others] he helped found the Hannibal Library Institute. A member
of the First Presbyterian Church--like Jane and Pamela Clemens--he
was also one of the church's elders. In the spring of 1849 he led a
party of Hannibal citizens to California and settled in Sacramento,
where he practiced law and eventually became a judge. Cross ran the
school Clemens attended in the mid-1840s, after instruction by
Elizabeth Horr and Mary Ann Newcomb. (Cross's older brother, William,
was also a Hannibal schoolteacher, though not one of Clemens's
instructors, as previously thought; see Wecter 1952, 131.) In an
autobiographical dictation of 15 August 1906, Clemens recalled the
"early days" when Hannibal had only two schools, both of them
private. "Mrs. Horr taught the children, in a small log house at the
southern end of Main Street; Mr. Sam Cross taught the young people of
larger growth in a frame school-house on the hill" (CU-MARK, in MTW,
107). Clemens mentions Cross only in passing in "Villagers" (97). His
working notes for "Schoolhouse Hill" (MSM, 436) show that he
re-created the physical setting of Cross's school--a frame house on
the public square facing Center Street, a "coasting hill"--in the
opening chapter of that story, although he based the schoolmaster
there on John D. Dawson . . . (p. 316, Indians).