Once, when I was a freshman in high school I covered my hoodie sleeve with my own snot
What was your reason for doing that?
It was a really hot day
It may not be a good idea to wipe your nose on your clothes since it will leave a stain. Also, this sleeve could touch other people and they could end up getting sick from your germs. You should do your best to keep paper handkerchiefs in your pocket whenever you feel you are going to have a runny nose.
It's just a stain, it washed out.
While I'm glad that your sweatshirt wasn't damaged, it sounds like this was a pretty inappropriate way to act at school or in public.  People might see you wiping your nose, or see your stained clothes and be offended.  It is a much better idea to carry some kind of handkerchief, and to blow your nose in private.