This is a phishing site. Do not put your information in it.
If you look at press releases from Xfinity/Comcast istelf, the only one about teachers is from August and is about new accounts.
Why would Xfinity need to ask for your information on a seperate website when you could just log in to your regular account and send them a note that you are a teacher?
Why would Xfinity need your birthdate?
This is a SCAM. Don't input your informaiton.
The website https://xfinityteacherreward.com/ is indeed a legitimate website. The offer expires on September 30. It is a new promotion which will be available for a limited time.
It does require you to enter the requested information in order to verify the submitter is an existing customer currently working as a teacher.