How do you describe this kind of student who is always hardworking, eager to learning things and never miss or be late for class. can i say he
/she is sincere student? or can i say the good student has sincerety.
I would say that in the context of learning, this is an exemplary student.
If you are looking for one word, I believe it may be "devotion" and as an adjective: devout.
de·vo·tionn. 1. Ardent, often selfless affection and dedication, as to a person or principle. See Synonyms at love. 2. Religious ardor or zeal; piety. 3. a. An act of religious observance or prayer, especially when private. Often used in the plural. b. devotions Prayers or religious texts: a book of devotions.
4. The act of devoting or the state of being devoted.
Also, zealot and zealous...but these are sometimes used as qualifiers that are both admirable and excessive.