How are the thirteen students grouped in the tug-of-war competition? Please set the grouping scheme.

In order to have a fair tug-of-war, under the premise of considering the same number of people in each group, we can consider drawing lots at random. Write the names of all the contestants on the note, then put these notes in a box, and one of them will draw lots. The names drawn in turn form a group, and finally form several groups, with the same number of people in each group as far as possible to ensure the fairness of the competition. If the number of participants cannot be evenly grouped, the upper limit can be calculated or the number of participants can be increased to the same number.