Answer to Question 1
Answer: The great person theory of leadership maintains that there are certain key personality traits or characteristics (e.g., intelligence, motivation) that make a person a good leader, regardless of the kind of situation confronting the leader; in other words, this theory posits that situations do not make a difference in who is a good leader. The contingency theory of leadership maintains that what makes a good leader depends both on characteristics of the leader (i.e., task-oriented versus relationship-oriented) and on the amount of control and influence the leader has over the group (to some extent, what types of leaders are more effective also depends on gender). Because the contingency theory takes into account both the person and the situation, most social psychologists prefer this theory.
Answer to Question 2
Answer: Your best bet is to address the antecedents of groupthink. Highly cohesive groups are more prone to groupthink, so you might ask to choose the members of the student group, and make sure to choose people who don't already know and like one another; this will reduce cohesiveness pressures a little. Isolated groups are more vulnerable, so you might invite outsiders with alternative points of view to participate in your discussions. Because groupthink is more likely in groups in which leaders are directive, you might wait until everyone else has spoken or made suggestions before you as leader make your viewpoint known. Groups operating under high stress (caused by perceptions of outside threat or strict deadlines) can be vulnerable to groupthink; encourage members to take their time and to remember that there is no enemy in this situation. Finally, it is important for you as a leader to standardize methods for considering alternative points of view; you might appoint a series of devil's advocates for each issue, and divide the group into subgroups who separately consider the issue and then bring their subgroup findings back to the larger group for consideration.