Because you start missing school. Your minds based on "what are they going to do/say to me next". Speaking from my own personal experience it made me shut down. Didn't talk to anyone, stopped turning in my homework and quit going. I think everyone is different in that situation.
When a person is being bullied its hars to concentrate on other things, like school. U dread going to school an u feel hopeless because u think no one will understand. Your grades tend to suffer an on top of all that depression has set in. Sad sad situation.
Your question makes sense. Bullying can affect your education because if your being bullied you most likely wont want to go to school and then you wont learn or you will be worried and think about your bully during class rather than doing your work.