I recommend logging into your School's website and double-check the district code. If you are able to log into the website, but not the app, this may be due to a capitalization error. The website is not case sensitive in the username field, but the app is. If your school supports resetting your password via the website, you can check your username in a similar fashion.
Here is a link to an article that gives you a detailed insight into this issue:
Error: Invalid Username or Password