In a classroom where there are only 20 students, which is a small number, the democratic form of governance that would make the most sense to use would be: Direct democracy.
Direct democracy is defined as the system of government in which the people have the direct election of the person, or people, who will represent them and they also have direct influence on the policies and decisions that the government and its representatives will have. It is also known as a pure democracy, in that there are no intermediaries between the people, who are the power behind the government, and the government itself. The people are, in reality, the decision-makers, and the ones on who final decisions are also based through direct vote for representatives. This type of system is very useful in small nations where the population, and landmass where they inhabit, is not so extensive that communication and contact between the government and the governed is impossible. This is why it would be a good way of governance for a small group of 20 students.