Perhaps you are incorrect and students are actually very careful and never litter... But if you are correct for some students, then it might be because...

# They don't think about other people who might have to clean up after them. # They haven't been taught what to do with their trash.

Their mothers clean up after them at home.

# They are too tired to move to a rubbish bin. # They are rebelling against the world and this is the safest way they can protest.

# There is no law against littering. # The law against littering is seldom if ever enforced. # They don't think recycling is important. # They think that just one more thing won't make any difference.