In order for a story to work, and for a reader to be able to relate to it, your characters need to be real, live, flesh-and-blood human beings. Too often in student writing, the characters are just names, and a name doesn't make a character. Nor does a person's age, gender, occupation, hometown; sure, these are facts that the reader may need to know, but they don't really reveal anything about the person.
Take a look at the following sentences:
A. Sheila replied, "I don't know where he is."
B. Sheila rolled her eyes and sighed loudly, then threw up her hands in exasperation and replied curtly, "I don't know where he is."
Sentence A reveals nothing about Sheila. It's a basic, simplex sentence with no descriptives; only a noun, verb and object (the quotation). Sentence B, on the other hand, while it still contains all the words and the same essential meaning of sentence A, reveals Sheila to be somewhat impatient, ill-tempered, authoritarian, perhaps even irritable.
C. Staring blankly out the window, Sheila exhaled softly, closed her eyes for a moment and replied, "I don't know where he is."
Again, we see the same noun, verb and object as sentences A and B, but Sheila-C is clearly a different person than Sheila-B. Sheila-C is calm, unemotional, distant, sedate. Notice, though, that as with sentence B, those characteristics are revealed rather than specified or explained.
D. Sheila stared at her mother teary-eyed, shuddering with fear, and chokingly replied, "I don't know where he is."
E. Sheila continued to pass the items over the scanner, staring down at the counter, and without even looking at the customer replied uncaringly, "I don't know where he is."
We now have five different Sheilas, each one different and unique, but in each sentence we have the same subject, verb and object. Sheila B is impatient and authoritarian; Sheila C is vacant and sedate; Sheila D is emotionally fragile and needy; Sheila E is lazy and incompetent. Sheila A is nobody; she's just a name, not a real person.
Try to make your characters into real people, not just names and biographies.