There could be kids in school who are sick and spread germs by coughing or sneezing on or around you. Many people get colds in the winter because they wear regular light clothes and go outside and aren't dressed for winter. They get very chilly, air goes through their clothes and down their throat and they start coughing everyday and it tickles their throat. Then they get very sick. Because people spread germs all the time when they cough and sneeze and when they touch just about everything they touch.
Germs are everywhere.
That's why it is important to always cover your nose and mouth when you have to sneeze or cough and remember to wash your hands frequently. Also scrapes and cuts that don't have Band-Aids will allow an infection to come into the body. The nose affects our taste cells and if the nose is blocked then the taste cells can't do their job. I think more people get colds in winter because they are inside more. The air inside houses and schools is often stale and has lots of viruses and other germs in it.