Check your financial aid or school tuition bill. Usually your student ID number pops up on a number of documents that the school uses to communicate with you, such as a financial aid bill. If you are a student who receives financial aid, your student ID number is likely to be on your award letter for the school year. This helps the school to identify who you are, along with your name, address and telephone number.
Check your letter of acceptance to the school or any letters following it. Usually right after your acceptance to a school, the university or college sends out letters with pertinent information. Your student ID number is likely to be on one or more of these documents, since the school uses these to keep track of students and differentiate between students with similar names and/or addresses.
Call your school's records department. Sometimes, these offices will give you your student ID number over the phone in case you forget or lose it. They will usually ask for other information to confirm that you are not someone else trying to obtain another student's information. Have pertinent information on hand that will help you identify yourself over the telephone, such as your name, your Social Security number and/or your address.
Log onto your student account online. Most schools have updated their student account systems to allow students to log on and manage their own accounts. Usually, these accounts require user names and passwords to access certain information and don't require you to input your student ID number. Your student ID number should be available on a website such as this one. You should receive information via mail about any online student account system available to you that the school has in place. Schools may have clever names for such systems, which may reflect the name of the school.