"Dear" can be interpreted to me someone very close in a friendly or non-romantic way. Dear sister, dear teacher, dear cousin could all indicate that the person is "dear" to you because they have been very kind or very helpful. "Darling" most often is used in a romantic sense, unless it is a very flamboyant person who addresses everyone as "darling" just as an affectation. (Think of Zsa-Zsa Gabor sweeping into a crowded room and greeting total strangers with "Hello, darling!") Sometimes doting family members will refer to one another with "darling" ("How is my darling granddaughter?"), but that term of endearment would rarely be used to address a teacher or non-family member or non-sweetheart.
Student or Learner