Stand in line is the correct answer because it means to wait in an orderly row with other people until it is your turn.

This phrase is common in American English at school, in stores, at bus stops, and at ticket counters. It teaches both the action and the idea of waiting politely.

Examples: Please stand in line before entering the classroom. We had to stand in line for twenty minutes to buy tickets.

The other choices are incorrect because go to the classroom means to move to a room, and sit side by side means to sit next to another person, not to wait in a row.