"Преподаватель" is more often a professor in college or university, or instructor on some specific courses. While you can list (and consider) teachers in school as "преподаватели" the reverse is not true - nobody calls anyone "учитель" in system of higher education.
Also, "учитель" is uses for describing someone who is a spiritual teacher. Sort of like Yoda was for jedis or Splinter for teenage mutant ninja turtles ;)
Another subtle thing worth to mention, on can refer to somebody as "учитель" (like in phrase
"Но учитель, как же мне тогда быть"), but referring to somebody as "преподаватель" sounds very unnatural to the native speaker. One will use the reference to position (like
"Профессор, скажите пожалуйста") or just name (
"Оксана Николаевна, можно задать вам вопрос?").