Im an 11th grader but Im supposed to be in 12th. I was held back because I was missing 3 credits. How do I earn enough credits to be in my right grade and graduate on time?

I can't say for every school, but the high school I went to offered summer class, and night classes. You may be able to take a night class for the first term to get those extra credits... I'm not sure how many you would neede for 3 credits, but it doesn't hurt to talk to a guidance counselor and ask about it. In fact, they can probably give you a lot better information, as they know about the school you're in.