They tell you that you're smart because you are.

You process differently. Now, here's the dealio. You need to come up with a plan to get your mind in gear. Passing grades is your goal, or you could make it higher.

What's something you like to do? Draw?
Limit yourself to one doodle per assignment, then allow yourself to doodle all over it when you're done.
Listening to music?
Allow yourself to only listen to one song per... Mini lecture that a teacher gives? Till yöu learn one new thing?
Basically set limits for yourself so you can work on school.

I'm always here if you need help with your work.

Try to get a study buddy, or multiple, so that you can have someone working with you.
See if your parents will let you get a tutor.
Try finding an online program that works for you, helps you remember things.
Use flash cards.

Most of all, believe in yourself! You can do this, so do it!
Go Corn, go Corn, go Corn!!!