A few of your employees have been hassling another emplolyee about her higher education. She recently started grad school and they make comments like, "College girl" or "You think you're better than us."
The tension is on the rise since she's obviously annoyed and has occasionally fought back saying, "I want to improve myself; it doesn't have anything to do with you!"
Discussion
Talk to them one on one and be direct — "I heard you hassling her about education. What is it about her search for a degree that bothers you?"
There are a variety of common responses: She might think she's better than me. She might want more money or different treatment from management once she achieves a higher degree. But in each case, their concern isn't about someone else, it's about themselves.
Your response to these critiques needs to be clear. Say something like, "Well, it seems unfair that because she's doing something positive in her life, other people are making fun of her. It doesn't befit our culture at this company — it's admirable to put in this hard work and effort. In this environment, I want to urge you to support each other — look inside, address your feelings internally, and knock it off."
There's good web-resources on the subject of education and the workforce. The CIO has an interesting article about the importance of education in the professions and some of the issues of workplace disparity.
You may also need to face the possibility that these other workers are hoping for more professional development through higher education or training. Discuss with them what programs your company offers to employees who want to seek additional education.