Why should you always wash your hands after holding your ball python?

It's not just after a snake.. you should wash your hands before and after handling any kind of reptile (turtles, snakes, etc.) 
This is because more than likely, the animal contains salmonella. (Be aware that anything the snake touches, salmonella could spread to IF the reptile has it.)

I hope this helps you and keep asking questions! (:

That's  a project for you to do. btw your teacher probably uses a plagiarism detector and if you do copy anything on this site she/ he will find out and instead of getting a good  or bad grade you'll  get a 0 no exceptions, and they'll  probably call ur parents. so do some research ask friends and fam to you.