Your question is tricky, because it's entirely possible for a student to live a “wild” lifestyle, yet still turn their homework in on time, and get good grades. Not all party animals bring home bad grades, and not all stoners fail to turn their homework in on time.
One term that sprang to my mind is goody two-shoes (although, I'll have to admit, that term is often used pejoratively). Many dictionaries define goody two-shoes to be an alternate form of goody-goody, which, according to Collins, is defined as:
goody-goody (noun) a smugly virtuous or sanctimonious person
However, at least Merriam-Webster allows for the term to be used in a more positive light:
goody two-shoes (noun) a person who is goody-goody; also : a person who is uncommonly good
If that doesn't work for you, though, you could use the describe this person as scholarly, or as a conscientious student.
Scholarly has a few different meanings, but I think one found in NOAD aligns with your purposes well:
scholarly (adj.) having or showing knowledge, learning, or devotion to academic pursuits
I would think that devoted to academic pursuits, in particular, might be a fitting description for people like Xiaoli. Conscientious also seems to be an apt word; NOAD defines it as:
conscientious (adj.) wishing to do what is right, esp. to do one's work or duty well and thoroughly
I think conscientious might be my best suggestion, because it carries connotations of both upright living and careful work habits. WordNet defines it as:
conscientious (adj.) 1 guided by or in accordance with conscience or sense of right and wrong
2 characterized by extreme care and great effort
I like Wiktionary's definition as well:
conscientious (adj.) Thorough, careful, or vigilant; implies a desire to do a task well