An I-20 is a 3-page document that is issued by your school. An Initial I-20 allows you to apply for an F1 student visa and enter the U.S. for the first time only.
Once you enter the U.S., check-in with a DSO, and enroll in classes at WT, you will be issued a Continued Attendance I-20. The Continued Attendance I-20 allows you to travel in and out of the U.S. (with a travel signature), apply for a driver's license, and more.
Your I-20 can NEVER expire while inside the U.S. If you will not finish your program by the end date listed on your I-20, you MUST apply for a program extension.