Are you really sure you are, like you say, "dumb", and that the best thing for you is that people just accept that fact and change the expectations and feelings they have towards you?
If you just feel like you are not very bright, and maybe that's a feeling your life experiences might be confirming... I am not saying you are wrong but don't jump to conclusions too fast.
Things like depression, anxiety, low-self esteem and learning disabilities can highly influence your school performance.
I'd suggest to get your cognitive abilities checked (even an IQ test is a good start) and your mental problems too.
Like someone else said it might just be ADD/ADHD, in that case I highly suggest drugs like amphetamines (Adderall, Dexedrine etc.) or methylphenidate. They work wonders for a lot of people. You take one pill and see if you can concentrate and be more productive, you don't have to wait week/months like with antidepressants.