There are a multitude of reasons why your memory could be affected...ADD, ADHD, vitamin deficiency, etc. You said something about school, so I'm assuming you're rather young. How old are you? There is an answer for you, and there are things to help you. Just contact your doctor--make an appt. Sometimes, it helps to make a list of your symptoms, and give examples of what is going on.
Ive heard the Herb Sage is suppose to help w/memory... Id def seek a professional if your memory is that bad could be like Joanna said. My memory isnt great but its help when I write stuff down.
I suggest that you see your Doc and get tested for ADD (Attention Deficit Disorder) There is most certainly help for your inability to remember (pay attention) things short term. See your doc, this is NOT uncommon.
Good Luck!
--J