Not very clear on what you are asking...but are you asking how to establish a student government?
If so, you should start by finding a group of people who are interested in establishing this student government (which is sounds like you have).
Then you should get this group to determine, with the input of the administration, what the role of the student government is. In general, that is usually to facilitate communication between the students and the school administration and faculty, to help lead and shape student run programs to help the students (like volunteer events, community outreach, tutoring or mentoring programs, study clubs, social events), and other student coordinated activities, fundraising.
Once you know what the student governments purpose is, you should create some roles for each class of students, define the jobs of these roles, and then hold elections for these positions:
Examples:
President - Act as voice for his/her class. Make decisions pertaining to activities and events and help to run and coordinate these events. Meet with Faculty, Admin, Staff, and students to coordinate communication.
Vice President - To act in the president's absence. To head up social events.
You could have every class have a president, vice president, etc. and all of them together make up the student government and meet together to create events.
You could also have it so only a couple positions are available in each class like:
President - senior class
Vice President - junior class
Secretary, Treasurer - sophmore class
Rep at Large, etc - Freshman class
Don't know if that's what you meant, but if not, please ask more clearly what it is you are trying to figure out.