Answer:
They may not know very much about you and assume things about you based on things they see and perceive that are separate from the grades that you get and/or they think you aren't intelligent on something to do with your emotions/behavior and/or someone or some amount of people have developed something against you, these are just potential situations.
Explanation:
Regardless of what the reason is, move forward with what you do and meet people who enjoy your company. Those who treat you badly are not worth your time. Best of luck!