i want to ask a question. those who know the answer pls tell me k! =D

as my working place is quite near river valley high school, very often i'll be in the same bus with them. hmmm... i noticed that each student has something like the rubber band that girls use to tie up their hair.. and each student has at least one hanging at their shoulder. somemore each is of a different colour: i saw blue, red and green.. cannot remember if i saw yellow before..

so very simple.. my question is what is that colour band on their shoulder? what does that symbolise? whats the meaning of each colour? what some have more than the others?

i can only guess that one band = sec 1.. two bands = sec 2....