There is nothing wrong from divesting from coal. It certainly is immoral to profit from the extraction of fossil fuels... But we ask these college students to just divest. Nothing happens. People just sell their stock.
[Other's disagree that divestiture tactics are not very useful. They do bring pubic attention to the fossil fuels threat. And divesture tends to drive the price of the coal companies down. That makes it harder to raise money to expand and mine more fossil fuels.]