Realize that most people who seem overly intelligent aren't any smarter than anyone else. It's often just a matter of presentation.
Recognize that you have your own kind of intelligence, which might not be measured well by standardized tests or "normal" grading criteria.
Tell yourself that you're not going to be intimidated by people who appear, in your eyes, to be smarter than you are.
Take the high ground: If someone tries to make you look stupid, realize it's probably that person's insecurities coming through and has nothing to do with your intelligence.
Study. Your feelings of intellectual inferiority may stem from being less prepared than your classmates, not less intelligent.
Contact your school's academic-counseling office. The professionals there have a wealth of advice for you.